This paper describes the adsorption of the unsaturated fatty acids, oleic-, linoleic-, and linolenic acid onto steel coated quartz crystal surfaces from 2,2,4,4,6,8,8-heptamethylnonane as monitored by the quartz crystal microbalance (QCM) technique. It is shown that addition of fatty acid to the oil results in changes in bulk density and viscosity and that these changes must be considered before the sensed mass can be evaluated. The change in viscosity of the solution is larger for oleic acid than for linoleic acid and linolenic acid, which results in a larger correction for oleic acid with respect to bulk effects. After considering the effects due to changes in bulk properties, the influence of the viscoelastic properties of the adsorbed layer on the sensed mass was evaluated. The correction for the viscoelastic properties of the adsorbed layer was found to be very small for the systems studied. The sensed mass, at 1.1 weight percent, ranged from 0.5 mg/m2 for oleic acid to 5 mg/m2 for linolenic acid.