A method for frequency response analysis of chemical reactors with periodically modulated inlet concentration is presented and demonstrated for simple first-order irreversible consecutive and parallel as well as reversible reactions applying an isothermal continuous stirred-tank reactor (CSTR). The frequency response of these reaction types is studied by simulations in time and frequency domain, the latter being based on Laplace transformation of the material balance. The results allow for clearly distinguish between the quasi and relaxed steady state as well as the full transient region with impact on designing reactors for dynamic operation. Furthermore, the relation between the dynamic response and the time constants of the system is discussed.