The research refers to the district heating (DH) system in Ljubljana, which includes 245 km of highly diversified pipelines. The purpose of this study was to determine the effect of the soil thermal conductivity coefficient (lambda(s)) on the heat loss from pre-insulated pipes during operation. Pipeline geometry, material properties (particularly insulation and soil thermal conductivity) and time-dependent data on supply, return and environs temperature were considered. Measurements of temperature, moisture, thermal conductivity of soil and heat flux through the soil were carried out at the chosen locations on the pre-insulated pipeline. In addition, laboratory measurements were conducted on the soil samples from this site in order to determine the soil density, specific heat and thermal diffusivity. For the evaluation of heat loss, transient and steady-state numerical simulations of the soil temperature field were performed. In transient simulations, in addition to the impact of environment temperature, the influence of supply and return temperatures was taken into account. A method for lambda(s) prediction during pipeline operation is presented. The algorithm is based on a comparison of the measured (Theta(exp)) and simulated (Theta(sim)) temperatures in the selected period of time with the same boundary conditions. (c) 2012 Elsevier Ltd. All rights reserved.