By particularly stressing the weaker labour market attachment of workers with non-standard contracts, this article contributes to the rather unexplored issue of mainly non-union-related reasons for leaving trade unions. Germany has been selected as a case study because German unions experienced a steady decline in membership, while at the same time non-standard employment arrangements increased considerably and more so than the European average. Using the German Socio-Economic Panel data, the authors construct a labour market attachment variable capturing different degrees of attachment. Their analysis shows the impact of labour market attachment and firm-level characteristics on union leaving and points especially to important differences across gender.