Background: Despite a lack of consensus regarding prevalence rates of substance abuse, people with intellectual disabilities (ID) on average use substances slightly less often than their non-disabled peers. However, their use of substances is more often problematic. Avoidance of treatment is a crucial problem among substance-abusing people with ID. This study tested a motivational intervention to facilitate autonomous motivation (i.e. wanting to change substance abuse because of a sense of free choice and volition) for engaging with a subsequent addiction treatment. Method: A multiple-case experimental design (N=6) was conducted to measure day-to-day motivation to change substance abuse among individuals with mild ID (N=3) and borderline level of intellectual functioning (N=3) in the Netherlands. The participants (five men, one woman) lived in the community (except one, he lived in a residential facility) and abused cannabis, alcohol or hashish. During the intervention phase, the 10-session treatment programme 'Beat the kick' was delivered by an experienced psychologist, based on motivational interviewing techniques adapted for people with mild to borderline ID. Participants completed an adaptive self-reported inventory based on self-determination theory (SDT) two to three times a week during baseline, intervention and 1-month follow-up. Results: The results of five of the six participants (one dropped out) showed that the type of motivation changed from more controlled types of motivation (i.e. external motivation and introjected motivation) at baseline to more autonomous types of motivation after completion of the intervention. In addition, the participants reported a significant increase in overall need satisfaction and autonomy satisfaction and a significant decrease of overall need frustration. Conclusions: The implementation of SDT and motivational interviewing principles in the treatment programme 'Beat the kick' reliably changed the type of motivation. In addition, the experimental effects provide initial proof of the use and applicability of SDT among people with ID.