The effect of O-2/CO2 = 90/10 gas mixtures under pressurized conditions on the diffusion resistance during oxygen reduction was investigated in carbonate melts of variable composition ranging from the eutectics Li-K, Li-Na to ternary Li-Na-K carbonate melts. It was found that pressurization reduced the diffusion resistance of the reactant species in all compositions investigated. This was ascribed to the increase in concentration of the reactive species in the electrolyte. Reaction order plots of Warburg coefficient versus the total pressure in binary melts showed that: (i) mixed diffusion of superoxide ions and CO2 prevails in lithium rich carbonates, and (ii) mixed diffusion of peroxide ions and CO2 predominates in potassium rich carbonates. By means of a computerized statistical analysis of the results, a partial cubic model was found to describe the relationship between alkali carbonate compositions, pressure, and Warburg coefficient. The optimum alkali carbonate compositions are: (i) 0.283Li-0.373Na-0.344 K with sigma(app) equal to 137.68 Omega cm(2)s(-0.5) at atmospheric conditions, and (ii) 0.390Li-0.355 Na-0.255 K with sigma(app) 87.53 Omega cm(2) s(-0.5) and 4.2 atm under pressurized conditions.