Mass transfer and kinetics of nitrification using free and alginate gel immobilized Nitrosomonas cells were investigated under no cell growth conditions. A simulated wastewater with industrial strength ammonia content was employed. Under the conditions employed, external diffusion limitations were negligible, but internal mass transfer was found to be very significant. The estimated effective diffusivity of oxygen in alginate gel was around 8.7 x 10(-9) m(2) s(-1) for assumed first order kinetics at 25degreesC. This value is somewhat lower than reported values, and the discrepancy was explained in terms of different conditions and presence of biomass in this study. The activation energy for nitrification with free Nitrosomonas cells was a-round 73 kJ mol(-1) and compared favourably with reported values. However, the Arrhenius plot for nitrification using immobilized cells displayed a break at 20degreesC, and this gave rise to two different activation energies. Between 20 and 35 degreesC, the activation energy was around 56 kJ mol(-1), whilst between 10 and 20degreesC, the activation energy was around 163 kJ mol-1. This result suggests that different rate limiting steps for nitrification using immobilized cells may apply according to the range of temperatures employed. (C) 2004 Elsevier Ltd. All rights reserved.