Abstract Lack of motivation for the treatment of drug addiction is associated with dropout and relapses. Further, personality disorders (PD) have traditionally been linked to low motivation and therapeutic failure. Thus, the present study aims to analyze the structure of the Motivation for Treatment Questionnaire (MTQ–8), as well as to determine differences in motivation due to the presence of PD and the impact of psychological adjustment on motivation. The sample included 125 patients (84% male) who started a treatment for their addiction to cocaine and alcohol. Rasch analysis was applied for the first objective, and means contrast and regression analysis for the others. The two subscales of the MTQ–8 fit the Rasch model, with appropriate psychometric characteristics when merging Items 5 and 7. The presence of PD was not associated with reduced motivation. Motivation for treatment was greater when abstinence was less than three weeks, and psychological distress predicted motivation for treatment. The present study confirms that MTQ–8 subscales are suitable for measuring motivation for treatment and readiness for change in drug-dependent patients. It is noted that the presence of PD should not be associated with a lower level of motivation, and that psychological distress influences motivation.