The environmental persistence of sulfanilamide (SFA) has become a matter of growing concern, underscoring the need for robust analytical tools to elucidate its environmental fate. Compound-specific isotope analysis (CSIA) represents a powerful approach for tracing transformation processes; however, its application to SFA remains challenging due to the compound's high polarity and limited volatility. In this study, a solid-phase extraction (SPE) method, combined with optimized derivatization procedures, was developed to enable reliable determination of stable carbon and nitrogen isotope compositions in SFA. Direct injection allowed carbon isotope analysis, resulting in a linear response at on-column carbon amounts exceeding 1.0 nmol. For nitrogen isotope analysis, derivatization with trifluoroacetic anhydride achieved complete trifluoroacetylation of the amino group, thereby enabling nitrogen isotope analysis with a linear response at on-column nitrogen amounts greater than 3.0 nmol. The method was successfully applied to experimental water samples containing SFA, which were subjected to simulated solar irradiation to evaluate the transformation mechanism. These results provide a theoretical basis for distinguishing SFA transformation pathways in natural waters and underscore the applicability of CSIA for investigating transformation processes in contaminated aquatic environments.