This paper considers whether unpaid overtime working relates to contracted working hours (i.e. whether an employee works part-time or full-time) in Britain. It uses the authoritative 2004 British data set Workplace Employment Relations Survey to derive a sample of 4,530 workers, from 735 workplace establishments, who worked unpaid overtime. It tests hypotheses linking contracted working hours to unpaid overtime, and whether this link is moderated by gender, occupational group and the availability of flexible working arrangements. Part-time workers were found to work significantly more unpaid overtime hours compared with their full-time counterparts. Gender, occupation and flexible working practices moderated this relationship, where the extent to which part-timers work more unpaid overtime than their full-time counterparts was greater for men than for women, was greater for professional/managerial part-time workers compared with other occupations, and was more evident in establishments less likely to offer flexible working arrangements. The findings raise concerns about the exploitation of part-time workers.