We conducted high‐temperature and high-pressure deformation experiments on hot-pressed aggregates of olivine + 9 wt% dolomite at 1 GPa and 1100ºC in a Griggs-type apparatus. At run conditions, the dolomite decomposed to produce ~7 vol% CO2 as a supercritical fluid. Microstructural observations show that CO2 is distributed at triple junctions and as isolated “bubbles” along grain boundaries in the hot-pressed samples. After deformation, the CO2 is redistributed to form CO2-rich bands and CO2-depleted bands. Compared to experiments with the basaltic melt, CO2 does not reduce olivine viscosity as much. The CPOs of CO2-bearing samples are much stronger than the CPO of melt-bearing samples, although [100] and [001] axes girdles are found in both types of samples. Olivine is much more soluble in basaltic melt than CO2, which enhances diffusion accommodated deformation mechanisms more in melt-bearing samples. Our mechanical and microstructural data support the hypothesis that the kinetic effect of a fluid or melt phase depends on the chemistry and fluid topology.