In order to understand the mechanism of fauling of electrode surfaces ofin vivo electrochemical sensors used for monitoring whole blood, the investigations on protein adsorption on the membrane surfaces were undertaken. Using an oxygen sensor, the protein adsorption studies were carried out on two different polymeric surfaces-teflon (PTFE) and polyurethane (PU)-using Scanning Electron Microscopy and Fourier Transform Infrared Spectrophotometry. Investigations reveal that even a single protein like albumin or fibrinogen tend to adhere to the sensor membrane surface and influences the electrochemical characteristics of the sensor. The influence is more significant, in the case of PTFE membrane.