A novel isothermal electrochemical microcalorimeter has been designed and tested. It shows a very fast response to temperature change at the working electrode surface, and can be used to monitor the cyclic voltammetric programme, the optimum sweep rate being 0.020 V s(-1). The cyclic voltammetric thermogram can be used to estimate the enthalpy change of an electrochemical reaction. A sensitivity factor of 3.012 x 10(-5) J mV(-1) is derived for the response of the Wheatstone bridge to the enthalpy change in a chemical reaction via the instantaneous measurement of temperature change at the working electrode surface. A one-dimensional semi-infinite diffusion model has been adopted and numerically simulated to describe qualitatively of the response function of the measuring system to the sweep rate in a cyclic voltammetric programme. A lock-in amplifier has been used in the Wheatstone bridge electric circuit and offers a high quality of amplification for a weak electric signal.