Assuming fast CO2 dissolution into the melt, the recombination reaction rate of O2- With CO2 was determined at 8.1 +/- 0.1 x 10(-2) atm(-1) s(-1) with oxide addition experiments. This is a few times higher than the value of 1.7 x 10(-2) atm(-1) s(-1) determined by Lu and Selman using the same experimental method [1]. Both values are six orders of magnitude smaller than the value of 7.6 x 10(4) atm(-1) s(-1) obtained by Nishina et al. from CV measurements [2]. A slow CO2 dissolution rate, though unlikely, was discussed as possible explanation for this discrepancy. For the fast and slow recombination reaction rates an acidic and basic melt was calculated respectively in the cathode in an operating MCFC. It was found here that the predominant reduced oxygen species in molten Li/K carbonates changes from peroxide in basic melts to probably superoxide in acidic melts, which is in accordance with most of the recent literature [2-10]. In the former case effects from autocatalytic reactions on the performance of the MCFC cathode as suggested by Adanuvor et al. are likely [11]. Non-equilibrium experiments showed that the cyclic voltammetry (CV) response of the reversible reduction of O-2(2-) to O2- in basic melts is characterised by the parameter gamma = c(O2-) root DO2-/cO(2)(2-) root DO22- which was determined here at gamma = 2.9 +/- 0.2/root pO(2) atm(-1/2). The equilibrium constants for 1/2O(2) + O2- double left right arrow O-2(2-) and CO32- double left right arrow CO2 + O2- were determined at 0.55 +/- 0.2 atm(-1/2) and 1.5 x 10(-8) atm respectively. The diffusion constant of peroxide at 4 +/- 2 x 10(-6) cm(2) s(-1). These values are in reasonable accordance with literature data [7, 8, 12].