The paper deals with the in situ experimental determination of the thermal properties of the underground soil for use in the design of borehole heat exchangers (BHE). The approach is based on recording the unsteady thermal response of a BHE that has been installed for the first time in Saudi Arabia. In this approach, the temperature of the circulating fluid has been recorded at the inlet and outlet sections of the BHE with time following the start of its operation. Severe fluctuations in these temperatures occur at small times (up to 8 h) due to the transient effects inside the borehole and must be excluded. A method has been developed for estimating the time period characterized by these severe fluctuations. The recorded thermal responses together with the line source theory are used to determine the thermal conductivity, thermal diffusivity and the steady-state equivalent thermal resistance of the underground soil. (C) 2009 Elsevier Ltd. All rights reserved.