A balanced calorimetric device has been developed to measure the absorption cross section of asinglemacroscopic sized particle (in this case a carbon fiber) at a microwave frequency of 35 GHz, as a function of size and orientation with respect to the electric field. The technique used is based on simple thermodynamic principles that relate the electromagnetic absorption of a particle to its thermal response. A single particle is irradiated in a fixed volume of gas by an approximate plane wave. Absorption by the particle results in a change in the ambient pressure that is proportional to the absorption cross section of the particle. Comparison of measured quantities with recently developed theory agree well.