Surface-enhanced Raman scattering (SERS) is of growing importance in different fields, from clinical analysis/chemistry to food industry. For a better insight into the complex light–matter interaction processes that are underlying the intensity enhancement, it is essential to experimentally distinguish and quantify the contributions based on the chemical mechanism (CM) with respect to the electromagnetic mechanism. Here, we present a model to estimate the relative CM response of Raman modes by analyzing light-induced degradation of target molecules on designed metal/semiconductor SERS substrates. The resulting intensity evolution is described by a biexponential function with two model parameters that allow a differentiation of the enhancement processes. Our work thereby provides a means for a better understanding of CM and will be advantageous for an application of intensity-based quantitative SERS techniques.