Activation of change processes in cognitive capacity of 71 institutionalized adults with mental retardation in four age groups was investigated. The central means of intervention was the Instrumental Enrichment Program. Effects of the intervention were examined in reference to logical thinking, predictive thinking, and insightful thinking. The test series was administered twice before and twice after the intervention. Although all subjects improved significantly, the initial and final scores of adults with Down syndrome were lower than those of adults with other forms of mental retardation. Subjects with moderate retardation achieved greater improvement than did those with mild retardation only in the Children Test. Significant changes were obtained in all age groups. For logical and predictive thinking, a divergency effect was found. Results support structural cognitive modifiability theory.