S-scheme photocatalysts have demonstrated great potential in solar fuel production. To study the electron transfer pathways in S-scheme heterojunctions, in situ irradiated X-ray photoelectron spectroscopy (ISIXPS) is an effective and widely used technology. However, the mechanism of ISIXPS in identifying the electron transfer pathways in S-scheme heterojunction has not yet been elucidated. In this Perspective, the development process and the formation mechanism of S-scheme photocatalysts are first introduced. Afterward, the principles of XPS and ISIXPS measurements are thoroughly explained, and the applications of XPS and ISIXPS in confirming the interfacial electron transfer in S-scheme heterojunctions are discussed. Finally, suggestions for future research on the utilization of ISIXPS in S-scheme heterojunctions are proposed. This Perspective will provide deep insight into the electron transfer mechanism in S-scheme photocatalysts through ISIXPS.