An external wall system for solar space heating and daylighting composed of transparent insulation material (TIM) and translucent phase change material (PCM) is presented. This system enables selective optical transmittance of solar radiation. Visible light is mainly transmitted and invisible radiation is mainly absorbed and converted to heat, causing in particular phase change. The storage medium is also the absorber. The concept of the system is presented in detail together with the investigations carried out, including a brief outline of modeling, optical experiments on PCR? samples and long-term experiments on a prototype wall as well as numerical simulations. The results indicate a promising thermal-optical behavior of the system. For instance in a Swiss lowland climate (Zurich-airport) a mean energy dux of 13 W m(-2) (system efficiency 0.27) was calculated through a south facing TIM-PCM wall into the building during the month with the lowest irradiation (December). The parameters of the prototype wall with a mean melting temperature of the PCM of 26.5 degrees C were assumed. When considering the percentage of time in which the building does not lose energy through the south facing TIM-PCM wall, a maximum can be reached with a mean melting temperature of approximately 20 to 21 degrees C. In this case energy losses through the facade occur only during 1% of the time. With regard to the practical application of the system in buildings, aspects of reliability and durability have to be further investigated.