This paper describes a modulated scanning calorimeter for studying, both in the isothermal and in the temperature scanning mode, liquids and solids that are chemically stable or undergoing structural transformations. The calorimeter concurrently measures, at a temperature modulation frequency,the complex heat capacity C-p* and the enthalpy change during chemical reactions or phase transitions. The time and temperature variation of the real and the imaginary parts of C-p* provide information on the molecular relaxation processes and the mutual influence between structure and dynamics in highly viscous samples as well as the glass transition. The principles and main features of the instrument and a set of experimental tests performed are described and accuracy, sensitivity, temporal stability and reproducibility of the data are given.