Unplanned and premature discharge from in-patient alcohol or drug detoxification is a common and severe problem in the treatment of substance abuse. So far, most of the relevant studies focused on drug detoxification, whereas only few studies also investigated alcohol detoxification. The aim of the present study was to comparatively identify and analyse determinants of unplanned discharge during in-patient treatment in both diagnostic groups which simultaneously underwent detoxification under the same treatment setting. Subjects were 239 consecutive admissions (alcohol: n = 90; illegal drugs: n = 149) to a specialised qualified detoxification unit at the Psychiatric University Hospital of Heidelberg during the year 2000. Data on sociodemographical and psychosocial variables, medical history, psychopathological findings on admission and presence of psychiatric and/or somatic comorbidity as well as intensity level of withdrawal symptoms were collected retrospectively and analysed with respect to the prediction of planned/unplanned discharge. The high overall rates of unplanned discharge (alcohol: 43.3 % and drugs 62.4 %) confirm the previously reported figures. Treatment success of drug patients was rather affected by sociodemographical and psychosocial factors such as level of education, delinquency, unemployment and hepatitis C diagnosis. Relating to alcohol patients psychopathological findings on admission including orientation, affective state and cognition were most relevant for planned discharge. Furthermore, the results of this study underline the central role of motivation during in-patient treatment as well as the importance of a planned treatment continuation after discharge from the detoxification program.