This dataset contains data from the moral registers of inmates who served a sentence at Arlon prison between 1870 and 1924. From 1870 onwards, the so-called 'moral accounting' became an established practice in the Belgian prison system. In these registers, prison authorities recorded information on the behaviour, family situation, level of education, offences committed, religion, and physical and mental condition of each convicted prisoner. This information served as the basis for decisions on pardons and early release.The dataset offers valuable information for a wide range of research purposes. For genealogical research, family members can learn more about ancestors who served a prison sentence. For local history, the dataset provides a unique insight into the social reality of Arlon and its surroundings at the turn of the twentieth century. Researchers working on crime and punishment will find rich sources for the study of criminal justice practices, prison policy, and the treatment of convicted persons in late nineteenth-century Belgium.This dataset was produced as part of project OUTLAW (2022–2026), a four-year citizen science project on prison archives. The project is a collaboration between the State Archives in Ghent and Ghent University, with the support of Histories vzw and funding from BELSPO.