The chemical stability and the thermal reliability of Phase Change Materials (PCMs), together with their lifespan, are key features in ensuring the economic feasibility of Latent Heat Thermal Energy Storage. While the thermal reliability of PCMs in opaque components is widely investigated, less information can be found in literature for PCMs directly exposed to solar radiation. This paper investigates, by means of different techniques, the thermal reliability and the evolution of the chemical-physical properties of a commercial-grade paraffin wax that was exposed to solar radiation during an outdoor test cell experiment on the prototype of a PCM glazing system. A quite good stability in terms of the latent heat of fusion after the ageing process is revealed and a decrease of the melting temperature range is recorded as time passes. The chemical structures of the PCM seem to not be significantly damaged by the ageing process, but a separation of two components in the materials that are initially well mixed together is recorded. A gradual formation of ordered domains can be detected. These facts may explain the change in the thermal behaviour of the paraffin: they may suggest that the small degradation caused by thermal cycling is essentially physical and that the influence of solar radiation is probably negligible, being the changes in the thermal properties in line with literature data on PCM subject only to thermal cycles. (C) 2016 Elsevier B.V. All rights reserved.