Physical chemistry I at Texas A&M University is an upper division course requiring mathematical and analytical skills. As such, this course poses a major problem for many Chemistry, Engineering, Biochemistry and Genetics majors. Comparisons between participants and non-participants in Supplemental Instruction for physical chemistry were made using analyses that controlled for prior mathematical ability and academic achievement. When controlling for prior mathematical ability, no statistical evidence was found that supplemental instruction attendance increased the final grade in physical chemistry I. However, when controlling for prior academic achievement, students with lower prior achievement were found to benefit from supplemental instruction while high achieving students derived no benefit.