The adsorption from benzene of a series of monodisperse poly(ethylene glycol) monododecyl ethers (C(12)E(n), n = 3, 5, and 8) and a polydisperse nonyl phenol ethoxylate with an average ethylene oxide chain length of 5 (N-5) Onto gold and silicon dioxide surfaces has been investigated in situ using quartz crystal microgravimetry (QCM). The frequency shifts caused by surfactant adsorption onto the QCM electrodes, as a function of the equilibrium surfactant concentration, cannot be described by Langmuir adsorption. The introduction of a concentration-dependent term to the frequency shift revealed a frequency dependence on surfactant concentration. This suggests contributions from the bulk solution properties. Hence, the linear relationship between the mass of surfactant adsorbed and frequency shift, implied the Sauerbrey equation frequently used in QCM experiments, could not be assumed. Complementary adsorption experiments onto colloidal silica enabled evaluation of the bulk solution contributions to the measured QCM frequency shifts. Corrected frequency shifts for C(12)E(3), C(12)E(5), and C(12)E(8) adsorbed onto SiO2 yielded a 1.15 Hz/ng calibration factor for the QCM, suggesting that the resonance frequency shifts in benzene are as predicted by the Sauerbrey equation for in-air measurements. For N-5 adsorbed onto SiO2, however, a calibration factor of 5.5 Hz/ng was obtained. It seems this increased sensitivity is due to interactions between the phenyl ring in N-5 and the benzene solvent. These calibration factors were used to calculate the surface coverage for each surfactant(and hence area per surfactant molecule) at the solid-benzene interface.