This study examines the reliability of the Motivational Interviewing Treatment Integrity (MITI) code, a brief scale designed to evaluate the integrity of the use of motivational interviewing (MI). Interactions between substance abuse counselors with one person role-playing a client were audiotaped and scored by trained teams of graduate and undergraduate students. Segments of 10 minutes and 20 minutes were compared and found to yield the same reliability and integrity results. Interrater reliability showed good-to-excellent results for each MITI item even with undergraduate raters. Correlations between items showed a coherent pattern of interitem correlations. The MITI is a good measure of treatment integrity for MI and seems superior to existing measures when indicators of client behavior are not needed.