ABSTRACT Steric effects strongly influence excited‐state electron transfer, yet accurately predicting their impact remains challenging. In this study, we quantified the steric bulk of 11 axial substituents on a silicon phthalocyanine photocatalyst using Sterimol parameters, and fluorescence‐quenching experiments were performed with N,N ‐diisopropylethylamine to evaluate the influence of steric effects on electron‐transfer efficiency. Multiparameter linear regression analysis revealed a correlation between the Sterimol parameters and the Stern–Volmer quenching constant ( K SV ), with distinct contributions from the Sterimol descriptors w B 1 , w B 5 , and w L. Among these descriptors, the minimum substituent width ( w B 1 ) was identified as the dominant steric parameter. Leave‐two‐out cross‐validation demonstrated reasonable predictive performance within the branching catalyst subset, highlighting the potential of this Sterimol‐based model to estimate K SV values for new structurally related silicon phthalocyanine photocatalysts. This work serves as a proof‐of‐concept for using Sterimol parameters to quantify steric effects in photocatalyst quenching and establish predictive structure–property relationships that may be extended to other photocatalyst classes.