There seems to be a general relation between the standard Gibbs energy change of unfolding, ΔG°unf, of a protein and its affinity to aqueous-solid interfaces. So-called "hard" proteins (ΔG°unf is large) are found to adsorb less strongly to such interfaces than "soft" proteins (ΔG°unf is small). Here, we provide direct support for this rule by using high pressure to modulate the folding stability of a protein. We have performed high-pressure total internal reflection fluorescence (HP-TIRF) spectroscopy and high-pressure neutron reflectometry (HP-NR) to measure the degree of adsorption and the structure of lysozyme on planar solid surfaces as a function of pressure for the first time. By carrying out these experiments at hydrophilic and hydrophobic surfaces with varying concentrations of glycerol, we have found strong evidence that ΔG°unf has indeed a direct influence. At high pressures, there is a larger degree of lysozyme adsorption, probably because lysozyme becomes a "soft" protein under these conditions. The results of this study demonstrate that high pressure is a very useful tool to explore thermodynamics of protein-interface interactions.