Background: Several studies have shown the positive effects of sex offender treatment for men with intellectual disabilities who have perpetrated sex offences or inappropriate sexual behaviour. The present study investigates the process of treatment change and compares two groups of offenders against adults and offenders against children. Method: Participants comprised of 15 men who had offended against adult women and 15 who offended against children. All were treated for 36 months. Groups were compared with repeated measures using the Questionnaire on Attitudes Consistent with Sexual Offending. Results: At baseline, offenders against women had higher scores than offenders against children on the Rape scale while on the offences against children scale the opposite was the case. Both groups showed significant improvement with scores reducing to levels consistent with non-offenders by 36 months. Significant improvements were still recorded from 18 to 36 months of treatment. Re-offending across both groups was 23%. Conclusion: Sex offender treatment can produce significant reductions in cognitive distortions in sex offenders with intellectual disabilities. The importance of continuing treatment for longer than 12 months was endorsed by the results.