This article discusses the use of ditolylmethane as the coolant in a functioning nuclear heat supply station (AST-1). Depending on the operating regime of the facility - the reactor power and the capacity of the purification system - the content of high-boiling (HB-) products in the coolant of the primary circuit was varied from 0 to 20%. Monitoring of the concentration of HB-products and of the viscosity of the coolant were performed regularly in all operating periods of the reactor. The measurement of the viscosity at 20 degrees C was conducted by a capillary method, using a standard certificated VPZh-1 viscosimeter. In order to determine C /SUB HB,/ the method of vacuum microdistillation was used.