Porous carbon materials were prepared by heating of a PET [poly(ethylene terephtalate)] and magnesium carbonate (3MgCO(3)center dot Mg(OH)(2)center dot 3H(2)O) mixture under Ar gas flow, followed by acid washing. The inorganic component of the mixture underwent thermal decomposition with formation of MgO and evolution of gaseous products including CO2 and H2O. Influence of the magnesium compound mixed with PET, and carbonization temperature on the pore structure of carbonaceous material is presented. As found, porosity of prepared materials depended on magnesium carbonate loading in the starting mixture and on heating temperature. The impact of CO2 and H2O evolved during heating on the pore structure is discussed, and a mechanism of pore formation is proposed.