In order to maintain the competitiveness and improve the environmental performance of district heating in the future, it is essential to transition to lower operating temperatures than those of current third generation district heating systems. Heat demands are expected to decrease as buildings become more energy efficient, and new types of heat sources are likely to be integrated. Therefore, future supply and return temperatures of 50-55 degrees C and 20-30 degrees C, have been suggested for the fourth-generation district heating systems. Previous studies on radiator temperatures are limited to the radiator's design conditions for modeling purposes, and there are no published studies that have established what the actual supply and return temperatures in radiator systems are. The purpose of this study is therefore to investigate what the temperatures are and to provide statistics on the radiator temperatures on the secondary side of district heating systems. The results, incorporating data from 109 radiator systems located in Gothenburg's district heating system, showed that the average supply and return temperatures for design outdoor temperature -16 degrees C are 64 degrees C and 42 degrees C, respectively. For an outdoor temperature of 5 degrees C or more all radiator systems had supply temperatures of 55 degrees C or less. The results also showed that radiator systems with a large radiator heat transfer area had the largest potential for lower radiator operating temperatures, as indicated by the normalized radiator constant and the specific space heating demand. (C) 2017 Elsevier Ltd. All rights reserved.