We investigated students' academic achievement in three high-enrollment, introductory-level history sections at a large, public, Hispanic-serving university. Using a conditional indirect-effects model, we analyzed supplemental instruction (SI) attendance and class absences as predictors of course success, after accounting for sex, ethnicity/race, and SAT/ACT scores. Results suggested a positive direct effect of SI attendance on course success and a negative direct effect of absences. A significant interaction effect between ethnicity/race and SI suggested that Hispanic students reaped stronger benefits from SI than Caucasian students, and that the course achievement gap between these groups was smaller when students attended more hours of SI. Our study contributes new findings to research supporting the effectiveness of SI by examining mediation and moderation effects and controlling for confounding variables.