The joint application of a thermal injection backflow test and an interwell tracer test for an in-situ determination of geothermal reservoir flow and heat transport parameters is proposed. The procedure is unique in several aspects. First, it uses two tracers with totally different transport characteristics, i.e. heat and a chemical/radioactive tracer, in a single field test operation. During field application of the proposed test, a low-temperature tracer carrying fluid is injected into the reservoir until an adequate amount of the chemical/radioactive tracer is recovered at the observation well(s). The tracer return profiles are interpreted to determine connectivity and flow velocity between-the injector and producer wells. As the thermal front does not travel far from,the injection well, the temperature transients during backflow are used to determine the heat transport parameters. The most important parameters controlling thermal breakthrough during long-term injection are, therefore, inferred in situ in a single field test. New analytical models are presented for interpreting the temperature and concentration profiles during the backflow period, providing a valuable insight into the collective roles of the parameters controlling heat transport in a single fracture/matrix system. (C) 2004 CNR. Published by Elsevier Ltd. All rights reserved.