Inter-well tracer tests were conducted at the Soultz-sous-Forets (France) site of the European Hot Dry Rock Program at less than 3900 m depth before the year 2000, and at about 5000 m depth in the period 2000-2005. The paper discusses the results and conclusions drawn from tracer tests performed in 2000-2005 during hydraulic stimulation operations and a short-term circulation test in wells GPK-2, GPK-3 and GPK-4. The tracer tests were conducted using organic compounds such as Na-benzoate, 1,5-, 2,7-, 1,6- and 2,6-naphthalene disulfonate (nds), and fluorescein. These studies indicated that the tracers behaved conservatively during a long period (at least 5 months for fluorescein and up to 4 years for Na-benzoate and 1,5-nds) under the harsh conditions of the Soultz reservoir (up to 200 degrees C and high fluid salinity). Combined with the monitoring of conservative species such as dissolved chloride, the tracer tests allowed us to estimate the fractions of native geothermal brine and injected fresh water in the fluids discharged during the short-term production tests. These tests consistently indicated that only low amounts of the injected fresh water were recovered and that the proportion of native brine was relatively high in the produced fluids. It was estimated that more than 750,000 m(3) of native brine mixed with approximately 142,000 m(3) of fresh water that had been injected since 2000. The mean natural flux for the native brine was estimated at about 1-1.2 m(3)/h. The tracer tests gave evidence of a fast and relatively direct hydraulic connection between GPK-3 and GPK-2 (short loop) but also indicated the existence of another larger and slower hydraulic connection between the wells (large loop). Hydrodynamic characteristics (mean transfer time, swept volume, tracer recovery, and mean fluid velocity) were estimated for both circulation loops. The hydraulic connection between GPK-3 and GPK-4 appeared to be poor. Differences observed between experimental and modeled data at the end of the fluorescein-recovery curves suggest the existence of another quasi-infinite loop connecting both GPK-2 and GPK-4 to GPK-3. (c) 2006 CNR. Published by Elsevier Ltd. All rights reserved.