Steady-state and transient measurements have been made at a rotating gold disk electrode in lithium carbonate at 800-degrees-C for simultaneously determining the diffusion coefficient and bulk concentration of peroxide ions. At high O2/CO2 ratios where these measurements were made, the diffusion coefficient is found to be essentially independent of concentration with a mean value of 3.1 . 10(-6) cm2 s-1. The bulk concentration of peroxide is two orders of magnitude higher than estimates based on thermodynamic calculations, but in agreement with values derived from oxygen solubility measurements in the literature.