Human carbonic anhydrase II pseudo-wild type (HCAIIpwt) and two truncated variants were adsorbed to approximately 9 nm silica nanoparticles. Ellipsometry was used as an indirect measure of protein adsorption. The structural changes of adsorbed proteins were investigated with the use of circular dichroism (CD), intrinsic fluorescence, ANS binding ability and inhibitor binding capacity. It was found that the variants that were truncated at positions 5 and 17 in the N-terminal end attain a molten-globule-like state after interaction with the silica nanoparticles. In contrast, the more stable HCAIIpwt retained most of its native structure after 24 h adsorption to silica nanoparticles. The result suggests that surface induced unfolding may give rise to intermediates similar to those for unfolding induced by, for example GuHCl. Thus, the intermediate observed has some features of the molten globule.