We investigated the temperature and angular dependence of the irreversibility field, B irr , of the quasi-two-dimensional organic superconductors κ-(ET) 2 X with X = I 3 , Cu(NCS) 2 , and Cu[N(CN) 2 ]Br by means of ac-susceptibility measurements. We find a crossover from an exponential temperature dependence at low temperatures and high fields, B irr ∝ exp(−AT/T c ), to a power law close to T c , B irr ∝ (1−T/T c ) α , with α in the range of 1.3 to 2. Concomitantly, the angular dependence of B irr changes from two-dimensional (2D) to three-dimensional (3D) behavior, respectively. The exponential B irr (T) dependence is interpreted as the influence of the breakdown field of proximitycoupled superconducting ET layers on the pinning mechanism. The power law near T c is explained by the usual 3D vortex pinning.