This thesis is an investigation of oxygen transfer, from air to water, in a large bubble column reactor (BCR) and a three-phase fluidized bed reactor (TPFBR). The experimental equipment was designed and built at the School of Chemical Engineering at Ryerson Polytechnic University. The experimental data collected confirmed the existence of two mass transfer zones in the BCR and two or three zones, depending on the fluidizing velocities, in the TPFBR. A Microzones Model (MZM) for modeling oxygen transfer in those reactors has been developed as a modification of the Two-Zone Model (Alvarez-Cuenca, 1979) taking into consideration the local variation of the volumetric mass transfer coefficient with height. The final stage of this investigation was the estimation, based on the oxygen transfer results, of nitrification potential in TPFBRs. The oxygen transfer rate gathered in this investigation showed 20% ammonia removal from water. Higher ammonia removal can be expected at longer liquid-phase residence times, higher air pressures and at higher air superficial velocities than those used in this investigation.