In this paper spectroscopic data of a proteolytic enzyme adsorbed on solid-liquid interfaces are discussed. The experiments consisted of time-resolved and steady-state fluorescence of tryptophan residues and of circular dichroism (CD) which give information on the tertiary and secondary state of the protein, respectively. The spectroscopic properties are measured for the inhibited form of subtilisin 309 in situ on a hydrophilic silica surface and on a hydrophobic Teflon surface. The results are compared with those obtained for the protein in solution. In the case of fluorescence it is reasoned that the average excited-state lifetime and short internal rotation correlation times are indicative parameters for structural changes in the protein. The internal rotation is superimposed on the rotation of the adsorbed protein which is immobile on the fluorescence time scale. Fluorescence and CD both prove that the protein alters its conformation when it adsorbs at low surface coverage on hydrophobic Teflon particles. In that case the tryptophan fluorescence lifetime is shortened which is accompanied by an increase in the cu-helix content. At monolayer coverage the protein maintains its original structure, although minor changes in fluorophore dynamics occur. On hydrophilic silica particles the results from both techniques do not point in the same direction. The fluorescence was not affected, irrespective of the surface occupation, while the CD experiments show a decrease in alpha-helix content at low surface coverage.