A simple facile method, i.e., thermal decarbonation of ZnCO3hydroxides, wasused to prepare a series of pure ZnO photocatalysts with controlled crystallitesizes, particle sizes, and morphologies. The ZnCO3precursor was synthesizedby direct wet carbonation in the presence of growth-control additives, i.e.,organic solvents, surfactants, and low molecular weight polymers. The thermaldecarbonation allows for producing ZnO photocatalysts with sizes and shapesvarying from 80±20 nm nonporous rhombohedral nanoparticles to 5±0.5lmporous particles, for a constant crystallite size of 64±3 nm. The porous ZnOparticles (5±0.5lm) exhibit two times larger photocatalytic activity formethanol oxidation than the nonporous ZnO nanoparticles (*180±30 nm).The reasons for the higher photocatalytic activity are further investigated in thiswork. A possible mechanism for the formation of ZnCO3hydroxides and theirtransformation into porous microsized ZnO particles and nonporous nanopar-ticles are carefully discussed.