The mean radiant temperature (T-mrt) is the most complex variable regarding the input parameters for heat balance models of human being that are the background for the assessment of thermally unfavourable conditions and heat stress. This paper investigates the simplification of past studies that the T-mrt is equal to the air temperature (T-a) under indoor conditions. In a second step, the causes for deviations between the two parameters are examined and integrated into the context of indoor climate. Measurements were conducted in four rooms at the Geography Department of Humboldt University in Berlin during autochthonal weather conditions from the 16th of August to the 2nd of September 2013. T-mrt was derived using integral radiation measurements and three different types of globe thermometers. The study indicates that the deviations between the different methods of obtaining T-mrt are negligible for indoor environments. The results show that the differences between T-a and T-mrt are negligible during most periods, as stated in previous literature. As air temperatures increase, however, T-mrt exceeds T-a up to 1.3 K. The examination of the surface temperatures indicates that rooms with window walls facing southeast and southwest show the largest disparities between T-a and T-mrt. The correlation between T-a and T-mrt and the sum of the short and long wave radiation specifies the radiation intensity and duration as the main driver of T-mrt. Future studies on indoor heat stress should hence consider that T-mrt and T-a can differ depending on the characteristics of the room and on solar radiation. (C) 2014 Elsevier Ltd. All rights reserved.