The mass transfer from the subducting lithosphere to the overlying mantle wedge is mediated by complex solutions resulting from dehydration and decarbonation processes. Compared to H2O-only and CO2-only fluid compositions, experiments dealing with mixed H2O-CO2 fluids in equilibrium with high-pressure minerals are limited. In order to investigate the speciation and the solute contents of COH fluids in equilibrium with mantle minerals we performed two sets of experiments at identical P, T and fO2 conditions using a rocking piston cylinder apparatus. Synthetic forsterite with minor enstatite was used as starting material. In the first set of experiments we investigate the composition of COH fluids by puncturing the capsule in a gas-tight PTFE vessel at T= 80 C. Evolved gases were convoyed toward a quadrupole mass spectrometer through a heated line to avoid the condensation of water. Oxalic acid dihydrate and graphite have been used to generate the COH fluid. Experiments were conducted at fO2-controlled conditions using NNO, OH (GX, COH) buffer and a double capsule technique. Following thermodynamic modeling we expect in our experiments fluids close to the binary H2O-CO2 join. The second set of experiments was performed to determine the solubility of forsterite in COH fluids. We analyzed the fluid trapped in a diamond layer by the cryogenic laser-ablation inductively coupled plasma-mass spectrometry technique [1]. COH fluids were generated from the addition of graphite, anhydrous oxalic acid and water. The results will highlight the importance of COH fluids for the mass transport in subduction zones. Comparisons with other experimental systems [2] and with thermodynamic calculations will also be shown.[1] Aerts et al.(2010) Am. Mineral. 95, 1523-1526.[2] Newton & Manning (2002) Geochim. Cosmochim. Ac. 66, 4165-4176.