Gas-phase axial dispersion (mixing of the composition of the gas phase along the longitudinal axis) was characterized in an enclosed spray tower for purposes of establishing reactor type for the solute-solvent pair oxygen and water. Test condition variables were spray tower height (TH), 1·52, 2·03 and 2·54 m; hydraulic loading (HL), 44·2, 66·3 and 88·4 kg/m²s; the ratio of volumetric oxygen injection to water flow rate (G/L), 1·0, 2·5 and 5·0%; the ratio of volumetric bulk tower gas recirculation flow rate to water flow rate (BG/L), 0, 500 and 700%; and bulk tower gas recirculation direction, counter-current to and co-current to the water flow. Gas composition measurements (% O₂) made across the long axis of the tower under steady-state conditions provided 1020 independent observations and 240 gas composition profiles. Factors showing a significant effect (P < 0·05) on gas composition were TH, HL, G/L and BG/L. Sample location as a percentage of TH did not have a significant effect on gas composition and accordingly profile slopes were not different from zero (P > 0·05). Profile data indicate a completely mixed gas phase within the tower. The dispersion observed was attributed to the lack of a significant pressure drop along the axis of the reaction vessel, forces due to nozzle operation, and to bulk tower gas recirculation.