This paper considers ignition and combustion of small (50–100 μm) single particles of magnesium and 50-50 magnesium-aluminum alloy in the atmosphere of carbon dioxide or its mixtures with argon. This investigation is of interest for both basic combustion science and applications to rocket engines, including those using Martian CO2 as an oxidizer, An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. A CO2 laser was used for heating to ignition of the particles. The laser was switched off after ignition. The experiments were conducted with the oxidizer at room temperature over the range of pressures from 0.1 to 2 MPa. Effects of the CO2 concentration and pressure on the critical ignition conditions, ignition delay times, and burning times have been determined for Mg particles. The results clearly indicate that ignition of Mg in CO2 is controlled by chemical kinetics and that its combustion is controlled by diffusion in gas phase. Quantitative disagreement of the observed critical ignition pressures with previous experimental data on ignition of Mg disks in CO2 is explained by the differences in heat-loss mechanisms. The measured values of the burning rate correlate well with previous experimental results on combustion of 2-mm particles and with a quasi-steady model of Mg particle burning in CO2. In contrast to pure Mg and Al, particles of Mg−Al alloy did not ignite in CO2 under the present experimental conditions.