An interior sun protection system consisting of vertical slats filled with phase change material (PCM) was monitored from winter 2008 until summer 2010. While conventional interior sun protection systems often heat up to temperatures of 40 degrees C or more, the monitoring results show that the surface temperature on the interior side of the PCM-filled slats hardly ever exceeded the PCM melting temperature of 28 degrees C even in case of long-term intense solar radiation. As long as the PCM is not fully melted, the latent heat storage effect reduces the solar heat gain coefficient (g-value) of the sun protection system to 0.25 for a totally closed blind, and 0.30 for slats set at 45 degrees (the g-values of the same system without PCM are 0.35 and 0.41, respectively). This reduced the maximum air temperature in the offices by up to 2K in contrast to a reference room with a comparable conventional blind. The sun protection system with PCM therefore considerably improves thermal comfort. In order to discharge the PCM, the stored heat must be dissipated during the night. In climates with sufficiently low outside air temperatures, this is best achieved using a ventilation system in combination with tilted windows. (C) 2011 Elsevier B.V. All rights reserved.