The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and Iiquidus phases of substituting C02 for H20 on a mole for mole basis at 30 kb, using olivine melilitite @ 20 wt% H20 at XcO2 = 0 and XCO~ --(CO2)/(H20 -}-COs) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite @ 38 wt% C02). Experiments were buffered by the 3/II-I or NNO buffers. At 30 kb, C02 is only slightly less soluble than water for XCO~ < 0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO 2 for H20 and at 30 kb, 1200 ~ C, Xco2 = XH~O = 0.5 the olivine melilitite contains 8.8 wt% t120 and 21 wt% CO 2 in solution. For Xco~-~I the COe saturated liquidus is depressed ~70 ~ C below the anhydrous liquidus and the magma dissolves approx. 17% C02 at 30kb, 1400 ~ C, Xco~-+ 1, XH,O-+0. Infrared spectra of quenched glasses have absorption bands characteristic of CO~ and OH-~ molecules and no evidence for I-ICOff. The effect of CO~ molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (< 5 %) of pyrolite at 30 kb, 1150-1200 ~ C, provided that both I-I20 and CO 2 are present in the source region in minor amounts. Preferred conditions are 0<Xco~<0.5, 0.5<XH~o<I, and at low oxygen fugacities (<Ni-NiO buffer). Residual phases are olivine, orthopyroxene, clinopyroxene and garnet.