Protein adsorption often plays the central role in a wide variety of processes occurring in medicine, biochemistry and biotechnology. In order to develop novel material coatings, a detailed insight into the underlying adsorption mechanisms at the molecular level and the knowledge of thermodynamic parameters is of great importance. So far, protein adsorption has been investigated in terms of concentration, pH-value and temperature of the protein solution. However, volume effects on protein adsorption are still unknown in spite of their fundamental contribution to protein-interface interactions. in order to investigate volume effects and protein conformational transitions, two model proteins, SNase and lysozyme, were adsorbed onto silica nanoparticles and subjected to high hydrostatic pressures. At pressures up to 2500 bar, we have observed much smaller volumes of protein unfolding in the adsorbed state as compared to the solution behavior. These changes are directly linked to volume changes upon protein adsorption at the aqueous-solid model interface.