Misreporting of adherence undermines detection of an association between product use and HIV infection in microbicide trials. This study investigates whether, in a placebo trial, audio computer-assisted self-interviewing (ACASI) produces more accurate reporting of adherence and sexual behavior than a face-to-face interview (FTFI). At three South African clinics, 849 women were enrolled and instructed to use applicators filled with placebo gel; participants were randomly assigned to FTFI or ACASI. Behavioral reports were validated through two biomarkers that detect product usage and unprotected sex. For most behaviors, ACASI generated significantly higher reporting, although differences by interview mode appeared to diminish over time. ACASI participants were more likely to report having had sex without gel, but reported and tested applicators did not indicate greater honesty about gel insertion with ACASI. While comparisons of reported unprotected sex with the validated biomarker revealed more agreement with ACASI than with FTFI, differences were small.