This paper describes how the quartz crystal microbalance can be used to estimate both the molecular area and percent ionization of Langmuir-Blodgett multilayers composed of fatty acids. This is done by doing a series of depositions of a variety of fatty acids (palmitic, stearic, arachidic, and behenic acid) onto the quartz crystal. The added mass decreases the vibrational frequency of the crystal. Relating the frequency change for each layer to the molecular weight of the fatty acid allows one to estimate the molecular area and the mass of any other inclusions in the multilayers. The method assumes that the different fatty acids have similar structure under the same deposition conditions. The assumption that the multilayers do not contain water allows the mass of the inclusions to be related to the mass of metal ions and thus to the percentage ionization of the fatty acid. The estimates of the molecular area and percent ionization, as determined using this procedure, are compared to data in the literature obtained by other techniques.