The formation and growth of Q-state CdSe particles during exposure of Langmuir-Blodgett films of cadmium arachidate (CdAr) to hydrogen selenide has been studied using UV-visible absorption, grazing angle FTIR, X-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. UV-visible absorbance measurements showed an optical blue shift typical of particles between 1.1 and 1.3 nn in radius. In addition to the formation of Q-state CdSe particles, elemental selenium (formed by the decomposition of H2Se) was present in the films. Grazing angle FTIR and XPS measurements (after subtraction of the contributions due to elemental selenium) were consistent with Q-state particle formation involving quantitative conversion of the Cd2+ ions in the films to CdSe and the corresponding conversion of CdAr to arachidic acid. Changes in the orientation of the surfactant molecules were also found to be a consequence of particle growth in the films. QCM measurements on films exposed to H2Se showed large apparent mass uptakes which were inconsistent with the trends observed using other techniques. This behavior was ascribed to anomalous interactions between H2Se gas and the QCM electrodes.