This paper presents the main results obtained during the geochemical monitoring of the fluids and deposits collected from the wells GPK-4 and GPK-3 after the four operations of chemical stimulation carried out in May and October 2006, and February 2007, at Soultz-sous-Forêts. During these operations, solutions of Regular Mud Acid (RMA), Nitrilo-Triacetic Acid (NTA) and Organic Clay Acid HT (OCA) were successively injected with fresh water into GPK-4 whereas only the OCA treatment was used for GPK-3. This paper shows that these acids have actively reacted with minerals present in the fractured and porous areas of the granite. The injection of the NTA basic solution (pH 12) into GPK-4 has allowed cleaning this well and a part of the fractured areas, and recovering significant amounts of drilling wastes (grease, rests of cuttings), rock fragments and hydrothermal deposits. Dissolved and solid Ca-Fe-NTA compounds were also formed. The combination of the RMA, NTA and OCA treatments is possibly at the origin of the significant increase of GPK-4 productivity. No major improvement in injectivity was observed for the well GPK-3 for which only the OCA treatment was used.