The effect of wastewater temperature on the rate of nitrification was studied in two pure-oxygen moving-bed biofilm reactors, fed on secondary effluent from a municipal wastewater treatment plant. The first Reactor (R1) was operated under ammonia-limiting conditions, while the second Reactor (R2) was operated under oxygen-limiting conditions. Quite surprisingly, the former showed a negligible influence of thermal changes on nitrification rates, while the latter showed a much higher dependence. In this paper, a temperature coefficient "theta" has been defined as the actual "intrinsic" biological temperature coefficient, similar to the corresponding coefficient that is usually adopted for the design of activated-sludge processes. In addition, an "apparent" coefficient theta(a) has been quantified independently, which was calculated according to the actual values of nitrification rates at different temperatures. The actual biological temperature coefficient "theta", ranged between 1.086 and 1.109 (average value 1.098) under ammonia-limiting conditions, while under oxygen-limiting conditions was in the range 1.023-1.081 (average value 1.058). The apparent value theta(a) was near to unity (i.e. no temperature effect) under ammonia-limiting conditions, while only under oxygen-limiting conditions and at constant dissolved oxygen concentration "theta(a)" coincided with "theta". An explanation was given that, under oxygen-limiting conditions, the specific biomass activity (i.e. the ratio of nitrification rate to biomass concentration) was strongly influenced by the combined effects of oxygen penetration through the biofilm and effluent temperature.