The conformational stability of surface immobilized protein monolayers is a key issue in applications requiring preservation of the protein bioactivity such as in biosensors and in vivo implants. Ellipsometry was used to detect conformational changes in a single monolayer of immobilized proteins on plasma polymer surfaces. The areal mass density of immobilized proteins was used to validate the data analysis in the protein denaturation analysis. We observed that the rate of conformation change was strongly dependent on the properties of the immobilized protein. Immobilized catalase showed a significantly slower denaturation rate than the immobilized horseradish peroxidase, indicating that the tetramer catalase is more stable than the immobilized monomer horseradish peroxidase at the surface/air interfaces. The ellipsometry results were in a good agreement with the enzyme activity analysis. (C) 2010 Elsevier B.V. All rights reserved.