The use of phase change materials (PCMs) for short-term heat storage in direct-gain passive solar applications is discussed. Approximate formulae are presented for optimum phase change temperature and the thickness of a PCM wall. Numerical simulations based on the Test Meteorological Years of Helsinki, Finland (60-degrees-N) and Madison, Wisconsin (43-degrees-N) indicate that a phase change temperature of 1-3-degrees-C above the average room temperature would yield optimal diurnal heat storage results. A desired phase change point can be accurately obtained by using fatty acids and their mixtures. To ease the installation, PCMs can be impregnated into conventional construction materials such as plasterboard. The thermal performance of a PCM wall in the direct-gain room in a residential application was briefly studied through hourly simulations. According to conservative estimates, direct energy savings of 5-20% could be expected, depending on climate. As this may not always be adequate for economic cost-effectiveness, the effect of increased thermal comfort plays also a key role in evaluating the total benefits of PCMs storage.