Infra-red light, generated by diodes, is applied to produce a sinusoidal temperature modulation in the standard heat-flux DSC of TA instruments. The temperature of the sample and reference are controlled by both, the original temperature controller of the instrument, and by the radiant energy generated by two high-power infra-red light-emitting diodes (LEDs). This allows for isothermal or quasi-isothermal experimentation in the standard mode of operation of the calorimeter. The additional infra-red irradiation is provided only for temperature modulation. The radiant energy transferred to the specimens is controlled by pulse-width modulation (PWM) which provides a high linearity in the temperature modulation program. This feature enables to obtain any arbitrary temperature profile with the limits of frequency set by the calorimeter response and speed of the computer. Simple electronics and a computer algorithm for the PWM control are described. The method was tested in the melting range of indium and by measurement of the heat capacity of sapphire (Al2O3) and polystyrene. The problem of frequency correction arising from a wide frequency range is analyzed and discussed. Published by Elsevier Science B. V.