A quartz crystal microbalance (QCM) was used to develop a real-time measurement method of airborne organic contamination on a silicon surface. At the relative humidity of 40-50%, the amount of adsorbed water on the silicon surface was determined to be several tens to 100 ng cm(-2). Under this condition, the adsorption and desorption of diethyl phthalate can be measured at the time corresponding to the operations of the diethyl phthalate vapor supply. Its adsorption rate and desorption rate are consistent with those evaluated in a clean room. Based on the weight evaluation of various contaminations, the significant part of the change in the surface concentration obtained by the QCM method in a clean room can be assigned to airborne organic contamination. (c) 2005 The Electrochemical Society. All rights reserved.