Transient depletion of source gases can play an important role in materials processing, particularly during the initial phase of thin film synthesis in which nucleation takes place and the interface is formed. In this paper, we present a zero-order analytical model that allows an estimation of the magnitude and timescale of transient depletion. The model is based on a lumped particle balance for a processing region and reactor volume that are coupled via a directive feed gas flow and diffusive transport. To illustrate the model, an experimental case study is presented on transient depletion during the parallel plate radiofrequency SiH4+ H2 plasma deposition of microcrystalline silicon for solar cells. The SiH4 steady-state depletion was experimentally determined by mass spectrometry, deposition rate and optical emission spectroscopy measurements. The transient depletion of the SiH4 was monitored by time-resolved optical emission spectroscopy measurements. Model and experiment are in good agreement. The implications for materials processing and thin film synthesis, as well as methods to control transient depletion, are discussed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschan.