This study examined the psychometric properties of the Group Engagement Measure (GEM) with a purposive sample of 61 adult male sex offenders in long-term, open-ended treatment groups. The GEM was highly internally consistent with a low standard error of measurement. Construct validity consisted of assessing the correlations between the GEM and the Group Attitude Scale, the Sex Offender Treatment Rating Scale, and the Facets of Sexual Offender Denial at the scale and subscale levels. The entire scale was favorably correlated with the constructs, and the subscales had mostly significant associations, with correlations ranging from mild to strong, depending on the measure or subscale.