This article examines two cases of successful efforts by UK trade unions to mobilise contingent workers. The evidence strongly illustrates the explanatory potential of Kelly’s mobilisation theory and deepens understanding of how mobilisation processes work within unions. The findings emphasise the importance of officers and activists in framing collective interests as ‘cultures of solidarity’ (Fantasia, 1988). Solidarity both within groups of contingent workers and between them and the wider union is essential for successful mobilisation. Building solidarities helps these workers overcome their inherently weak position in the labour market; a process that is important to understand in greater detail if we are to understand union revitalisation efforts more widely.