The nonlinear frequency response (NFR) method is used for evaluating the time-average performance of a chemical reactor subjected to single input modulations of general waveforms, by using Fourier series for representing the input and Volterra series for representing the output. Both the input and the output are approximated by finite sums. The obtained results are applied for the case of a square-wave input modulation. As a case study, the improvement of an isothermal continuous stirred-tank reactor with simple reaction mechanism with modulation of the inlet reactant concentration is used and the results are tested on a numerical example.