Zinc-containing solid catalysts (zinc oxide, smectite, hydrotalcite) and several inorganic zinc salts were used to produce glycerol carbonate from glycerol and urea under solvent-free conditions at 130 degrees C and at a reduced pressure of 3 kPa. The leaching of Zn species was observed to occur for the solid catalysts, and the carbonate yield was shown to be correlated with the amount of zinc species dissolved into the liquid phase with a single relationship in common for all the catalysts employed. The reaction was also indicated to continue in the liquid phase alone, after the solid catalysts were removed from the reaction mixtures by filtration. The results obtained reveal that the reaction takes place homogeneously in the liquid phase irrespective of the parent solid catalysts used. Possible structure of active Zn species was discussed from the results of reaction runs under different conditions and Fourier transform infrared spectroscopy measurements of the liquid phase after the reaction. (C) 2012 Elsevier Inc. All rights reserved.