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Rock–plant filter wetlands are a potential alternative to supplement onsite wastewater treatment systems. A plug–flow with dispersion (PF/D) model and an ideal plug–flow (IPF) model combined with first–order kinetics were tested for the prediction of treatment performance in rock–plant filters. Six meso–scale rock–plant filters growing narrow–leaf cattails (Typha augustifolia) were studied to verify the models with actual internal performance data. Tracer studies were conducted during summer, fall, and spring to estimate effective values of model parameters. First–order kinetics was applicable for organic matter removal, but the effective rate constant (KT,eff) values were consistently less than empirical design rate constant (KT,emp) values when plants were not dormant. The KT,eff in planted cells showed less temperature dependence than expected, in one case varying as little as from 0.0310 to 0.0324 h–1 throughout the year. This might have resulted from the offsetting effect of prolonged hydraulic retention time caused by evapotranspiration when temperature increased. Both IPF and PF/D models were reasonably accurate for performance prediction, but the increased complexity in the PF/D model resulted in little improvement in performance prediction. The accuracy of the IPF model was enhanced by using effective data derived from tracer studies, which reduced least square errors up to 90% compared to using the same model with empirical design values. Seasonal effects were not significant when using media of smaller particle size. Plants also contributed to the reduction in temperature dependence in treatment performance. The results of this study verified that the first–order IPF model could reasonably predict treatment performance.