Microfluidic systems have become important platforms for diagnostic and therapeutic applications. However, as channel dimensions decrease within these systems, the surface properties of these microchannels become increasingly important. When biological fluids are flowed through such channels, nonspecific protein adsorption can occur due to surface interactions, often resulting in reduced efficiency of the system. When small quantities of biological sample are involved, any loss of sample through the system can be critical. Poly(ethylene glycol) (PEG) films are known to reduce protein adsorption, but the existing approaches for coupling PEG to surfaces is not always appropriate for the micro/nanoscale features common in microfluidics systems. Therefore, we have developed a vapor deposition technique to modify microchannels with PEG. These films have been extensively characterized by contact angle measurement, x-ray photoelectron spectroscopy, and atomic force microscopy. Results show that film composition can be controlled by controlling the reactant concentrations and that the films are stable up to a period of 4 weeks. Moreover, protein adsorption is decreased by 80%. The vapor deposition technique may be more efficient in coating micro/nanosized features since it forms uniform, conformal, and ultrathin films on the surface. Surface modification using chemical vapor deposition of PEG shows promising results for a variety of bio-micro-electromechanical system applications. (C) 2003 American Vacuum Society.