Abstract We report on detailed investigations of the mixed state in the mercury based high- T c superconductor HgBa 2 CuO 4 (Hg-1201) by means of ac and dc magnetization measurements. Over large parts of the B – T phase diagram the vortex mobility turned out to be remarkably high. From knowledge of the phase boundaries and the melting line, in particular, it became clear that in Hg-1201 thermal activation strongly supports the flux diffusion in both liquid and solid vortex phases. Thermally assisted flux diffusion widely stops here only for temperatures well below the melting line. The strong decrease of flux mobility within the solid phase suggests a transition between different regimes originating from a different nature of vortex pinning, i.e. collective or single, and indicates a weak to strong pinning crossover of vortex matter in pure Hg-1201.