[en] Investigations of the properties of absorbed monolayers have received great experimental and theoretical attention recently, both because of the importance of surface processes in practical applications such as catalysis, and the importance of such systems to the understanding of the fundamentals of thermodynamics in two dimensions. We have adapted the composite bolometer technology to the construction of microcalorimeters. For these calorimeters, the adsorption substrate is an evaporated film deposited on one surface of an optically polished sapphire wafer. This approach has allowed us to make the first measurements of the heat capacity of submonolayer films of ⁴He adsorbed on metallic films. In contrast to measurements of ⁴He adsorbed on all other insulating substrates, we have shown that ⁴He on silver films occupies a two-dimensional gas phase over a broad range of coverages and temperatures. Our apparatus has been used to study the heat capacity of Indium flakes. CO multilayers, ⁴He adsorbed on sapphire and on Ag films and H₂ adsorbed on Ag films. The results are compared with appropriate theories. 68 refs., 19 figs