A description will be given of a new method to measure calorimetrically ac losses in superconducting samples. The boil-off gas is stored below the liquid helium level in a measuring tube. Losses in the range of 1 mW up to 100 mW can be measured with an accuracy of 3%. Results of measurements of ac losses in superconducting wires are presented. 8 figs., 1 tab., 3 refs.