Effective medium theories (EMTs) find widespread applications in the general area of condensed matter. In this article, EMTs are used to investigate the properties of thin films of silicon nitride (SiNx) and silicon oxide (SiOx) produced by the low-pressure chemical vapor deposition technique. Experimental data on the technologically important properties, such as the optical constants and density, of these materials are compared with theoretical predictions using Clausius–Mossotti, Maxwell-Garnett, and Bruggemann equations. For both materials, the results show a good agreement between theoretical predictions as obtained from the solution of the Bruggemann equation and the experimental data up to the percolation threshold. The experimental results agree fairly well with the calculated results obtained through the solution of the Maxwell-Garnett equation, but the latter fails to predict the transition present in the composition versus optical constant curves of both materials. © 2002 American Vacuum Society.