This paper presents some results, both practical and theoretical, from which the equivalent linearization approximation used in the analysis of two input non-linearities can be assessed. A two input equivalent linear gain is calculated for an exponential non-linearity. It is shown that this theory can be used to predict the amplitudes and frequencies of a stable, self-starting mode of oscillation containing two fundamental components at incommensurable frequencies if the amplitudes are equal. An explanation of difficulties previously encountered in this prediction is presented. † Communicated by Professor J. C. West.