High pressure is known to possess a modulating effect on protein aggregation - both in terms of inhibition and triggering of this process. It is not yet clear, though, which particular stage on an aggregation pathway is addressed and affected by pressure. Pressure Perturbation Calorimetry (PPC), Differential Scanning Calorimetry (DSC) and time-resolved FTIR have been employed to investigate aggregation of bovine insulin at low pH. As moderately high pressure of 30-40 MPa has been implicated to possess a strongly inhibiting effect on insulin aggregation, we were seeking to explain these changes monitoring volumetric effects on the aggregation pathway. PPC results show that the stage of actual, exothermic aggregation (seen in FTIR spectra as the appearance of non-native b-strands) is preceded by formation of the molten globule state and its unfolding. The unfolding is accompanied by a volume expansion of approx. 0.2 %. High pressure FTIR showed that under hyperbaric conditions the aggregation is blocked already at the first stage. Namely high-pressure prevents unfolding of insulin.