Thermal treatment of pure nanoscale maghemite (gamma-Fe2O3) powders under argon yields different products depending on particle size. Particles with a mean diameter above 15 nm transform to hematite (alpha-Fe2O3). Smaller particles, which were synthesised under identical conditions but from solutions with lower Fe3+ concentrations, transform into magnetite (Fe3O4) during thermal treatment. This phenomenon can be explained by adsorbed or incorporated reducing carboxylate groups. The origin of the reducing species and possible reaction mechanisms with the maghemite particles are discussed.