Abstract We have investigated the irreversibility line for bulk YBa 2Cu 3O 7:Y 2BaCuO 5 composites with varying mass ratios by performing AC susceptibility measurements. The irreversibility line depends on the concentration of the 211 inclusions demonstrating the influence of microstructure and pinning mechanisms. Assuming a general form for the pinning potential U p≈(1- t) α/ H β where t= T irr/ T c, the behaviour can be understood in terms of the giant flux creep model in which the irreversibility line follows a power law 1− t= aH q . Different values of a and q are found for each sample. Another important feature reported here is a change in the values of a and q in moving from a low to a high field region of the Applied AC magnetic field marking a crossover between two regimes. The frequency dependence of the irreversibility temperature T irr is also used to test the validity of flux creep and vortex glass models.