A gas‐filled microcalorimeter is described. This instrument can be used to measure the simultaneous work and heat phenomena during uniaxial straining or destraining of thin sheetlike materials. The average accuracy of the heat‐flow measurements of 0.1 mW or higher is about ± 10%. The same accuracy applies for heat measurements of 2 mJ or greater in the case of 1–2 min deformation experiments.