Conformational changes of bovine -lactalbumin induced by adsorption on a hydrophobic interface are studied by fluorescence and circular dichroism spectroscopy. Adsorption of bovine -lactalbumin on hydrophobic polystyrene nanospheres induces a non-native state of the protein, which is characterized by preserved secondary structure, lost tertiary structure, and release of calcium. This partially denatured state therefore resembles a molten globule state, which is an intermediate in the folding of bovine -lactalbumin. Stopped-flow fluorescence spectroscopy reveals two kinetic phases during adsorption with rate constants k1 ~ 50 s1 and k2 ~ 8 s1. The rate of partial unfolding is remarkably fast and even faster than unfolding induced by the addition of 5.4 M guanidinium hydrochloride to native -lactalbumin. The large unfolding rates exclude the possibility that unfolding of bovine -lactalbumin to the intermediate state occurs before adsorption takes place. Stopped-flow fluorescence anisotropy experiments show that adsorption of bovine -lactalbumin on polystyrene nanospheres occurs within the dead time (15 ms) of the experiment. This shows that the kinetic processes as determined by stopped-flow fluorescence spectroscopy are not affected by diffusion or association processes but are solely caused by unfolding of bovine -lactalbumin induced by adsorption on the polystyrene surface. A scheme is presented that incorporates the results obtained and describes the adsorption of bovine -lactalbumin.