The real-time adsorption and desorption of water and diethyl phthalate on silicon surfaces were studied using a quartz crystal microbalance in ambient nitrogen, having a humidity, average gas velocity, and diethyl phthalate concentration of 0-40%, 0.1-0.4 m s(-1), and 0.01-0.04 g m(-3), respectively. In the steady state, the amount of adsorbed water and diethyl phthalate is considered to depend only on each concentration in the ambient nitrogen, like that in a single-component system. However, in a transient state, the gas velocity at the quartz crystal microbalance influences both the adsorption rate and the desorption rate, which can change the abundance of organic compounds on the silicon surface in a multicomponent system. Therefore, the gas velocity at the measurement point is concluded to be the key parameter that influences the organic contamination measurement. (c) 2005 The Electrochemical Society. All rights reserved.