The spray Ion Layer Gas Reaction (ILGAR) is a well-established, patented and commercial process used primarily to deposit In2S3 as buffer layers in thin film solar cells. In this paper we investigate the growth mechanism of the spray In2S3 ILGAR process by characterising the intermediate growth stages of films, following the growth mechanism with a quartz crystal microbalance and tracking the gaseous side-and-intermediate products during film growth, using a mass spectrometer. A basic growth mechanism model is then proposed based on an aerosol assisted chemical vapour deposition of an In(O-x,Cl-y,(OH)(z)) film,as the first stage process, followed by the conversion of the intermediate film using H2S gas to In2S3. (C) 2011 Elsevier B.V. All rights reserved.