Efficient catalysts are of extraordinary importance for the development of efficient chemical processes as well as applications such as energy storage. Rational development of catalysts requires a mechanistic understanding of the catalytic reaction. Since steady-state investigations are insufficient to gain mechanistic understanding, transient methods such as SSITKA and frequency response have been developed. In this paper we provide a theoretical basis for a frequency response method based on variations in the isotopic composition of the reactant. This approach is particularly useful, since it permits transient investigations under quasi-steady state conditions and because the response is linear.