On pulsing CH4 over MgO containing various amounts of praseodymium oxide (PrOx) at 1023 K, the CH4 conversion decreased with increasing pulse number, and both the initial activity and selectivity to C-2 products (corresponding to the first pulse) decreased with increasing PrOx content. Characterization by XRD, SEM-EDX and XPS showed calcined materials to contain well-dispersed PrOx (x = 1.83-2) at low Pr concentrations, but only crystalline PrO1.83 at high (10 wt%) Pr concentration. A PrO1.83 phase was present at all Pr concentrations after the He treatment at 1023 K, and PrO1.83, PrO1.75 and PrO1.5 after reaction.