As the world grapples with a persistent issue of fossil fuel overconsumption, two prominent challenges have emerged on the global stage, i.e., energy crises and environmental pollution. These hurdles cast a shadow over humanity’s pursuit of sustainable development, underscoring an urgent need for renewable and clean energy sources. Among the array of technologies aiming to tackle these challenges, semiconductor-based photocatalysis technology shines as a promising avenue. This technology holds a key to harnessing solar energy and converting it into chemical energy, offering a promising application potential. However, the widespread adoption of this technology is hindered due to the inefficiency of single-component photocatalysts. This inefficiency is fundamentally since photogenerated electron-hole pairs within these single-component photocatalysts recombine readily.Some heterojunction photocatalysts are developed to address this limitation. Constructing such photocatalysts is considered as a pivotal approach to preventing the recombination of photogenerated electron-hole pairs, thus enhancing the photocatalytic performance. Recent emergence of S-scheme heterojunction photocatalysts, featuring reduction photocatalysts (RP) and oxidation photocatalysts (OP), is an effective strategy for averting the recombination of these electron-hole pairs.The conduction band (CB) and Fermi level (Ef) positions of RP surpass those of OP. When OP intimately contacts RP, electrons from RP migrate to OP through the interface until equilibrium is reached. This leads to an upward bending of the energy band near the RP interface due to electron depletion. Conversely, energy bands near the OP interface bend downward due to electron accumulation. This creates a built-in electric field (IEF) at the interface, pointing from RP to OP. Furthermore, the Ef of RP gradually decreases from RP's bulk to the interface, while the Ef of OP gradually increases from OP's bulk to their interface, until they meet at the same point. Under irradiation, electrons in OP and RP are excited from their valence band (VB) to CB, respectively. The built-in electric field at the interface propels the transfer of photogenerated electrons in the CB of OP to the VB of RP. Simultaneously, Coulombic repulsion, band bending, and the built-in electric field inhibit the transfer of electrons from the CB of RP to the CB of OP (i.e., hole transfer from VB of OP to VB of RP). In this scenario, the original high reduction ability of photogenerated electrons in RP remains, and the original high oxidation ability of photogenerated holes in OP preserves. Consequently, this heterojunction ensures an efficient charge separation and augments the redox capabilities of charge carriers, ultimately enhancing a photocatalytic performance. From a macroscopic perspective, the electron transfer is akin to ascending a “staircase”. Accurately characterizing electron transfer at the interface of S-scheme heterojunctions is vital for understanding photocatalytic mechanisms and for providing experimental and theoretical guidance for the preparation of high-efficiency photocatalysts.Recent methods are developed to investigate the charge transfer behavior in S-scheme heterojunctions. These strategies are categorized into direct and indirect verification of S-scheme heterojunctions. Direct methods include in-situ irradiation X-ray photoelectron spectroscopy (ISI-XPS), zeta potential measurements, and femtosecond transient absorption spectroscopy (fs-TAS). ISI-XPS and surface potential measurements directly evaluate electron accumulation or depletion by assessing relative energy shifts and electrostatic surface charge distribution. In contrast, fs-TAS tracks ultrafast electron transfer processes at the RP/OP interface. Indirect methods encompass work function measurements, electron paramagnetic resonance techniques (EPR), selective deposition of metal nanoparticles, and photocatalytic reactions. These indirect or complementary methods provide the validation of charge transfer behavior within S-scheme heterojunctions.Summary and prospects This review represented various methods used to explore the electron transfer mechanism within S-scheme heterojunctions. Each method was introduced with a fundamental explanation of the mechanism, followed by practical application examples. Some challenges persisted in elucidating the electron transfer mechanism within S-scheme heterojunctions. Despite the progress in characterization techniques for electronic transfer mechanisms, understanding the S-type heterojunctions and optimizing the photocatalytic systems need some techniques with atomic-scale resolution to analyze the dynamics of photogenerated charge carriers from a microscopic perspective. Such techniques, like ISI-XPS, zeta potential measurements and fs-TAS, should be complemented by advanced methods for in-situ characterization, providing research avenues for understanding the photocatalytic mechanisms of S-scheme heterojunctions in solar energy conversion.