Ammonia-removal efficiency in constructed wetlands is often limited by the amount of oxygen available in the system. In this study, the oxygen-transfer rates to laboratory-scale, subsurface-flow and free-water-surface constructed wetlands were quantified for two input concentrations of ammonia, 10 and 50 mg nitrogen/L, both with and without Typha latifolia. Rates of oxygen diffusion into the subsurface-flow constructed wetlands were 6.01 to 7.92 g/m2 x d and were strongly correlated with ammonia concentration in the water. In contrast, low oxygen diffusion rates of 1.36 to 1.76 g/m2 x d were observed for the free-water-surface system. The presence of Typha latifolia had small but consistent effects on atmospheric oxygen-diffusion rates and ammonia-removal efficiencies in both the subsurface-flow and free-water-surface systems. The observed small effects were consistent with measurements of oxygen inputs from T. latifolia roots. Only approximately 0.023 g/m2 x d of oxygen was released from roots of T. latifolia, as measured using the titanium (III) citrate buffer method. The results for the ammonia-removal rates of the laboratory-scale constructed wetlands were further verified using pilot-scale constructed wetlands under greenhouse conditions.