The most widely implemented offending behaviour programme in the United Kingdom was Enhanced Thinking Skills (ETS), a cognitive-behavioural group intervention that aimed to develop participant’s general cognitive skills. A new offending behaviour programme has been developed to replace ETS: the Thinking Skills Programme (TSP). This study reports an evaluation of the effectiveness of TSP using psychometric assessments. Phasing of the two programmes created an opportunity to compare the two programmes consecutively. Forty participants, 20 from each programme, completed a range of psychometric measures to examine cognition, attitudes, and thinking styles. Analysis of pre- and post-programme psychometric results indicated that participants of TSP demonstrated improvements on 14 of the 15 scales, 9 of which were statistically significant. Effect sizes between pre–post results were generally greater for TSP than ETS, demonstrating that TSP had a more positive impact on the thinking styles and attitudes of participants than the ETS programme.