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Data were collected from a top loading vertical flow submerged bed constructed wetland treating primary domestic sewage effluent to evaluate the performance of the system. The treatment wetland, located in Highland, NY, consists of four, 237-m2 beds, which may be operated in parallel or as four beds in series. Two beds were planted with Reed Canarygrass (Phalaris arundinacae) and two with common reed (Phragmites communis). Current operation is four beds in series with a mean flow rate of 7.5 m3 d-1. SPDES permit levels are: BOD5, 5 mg L-1; TSS, 10 mg L-1; NH4+, 2.2 mg L-1; and PO43-, <1 mg L-1; respectively. Redox potentials were measured with platinum tipped probes installed at 15, 30, and either 45 or 60-cm-depth within each bed. System influent and effluent samples from each of the four treatment units were analyzed for CBOD5, NH4+, NO2-, and NO3-. Mean wastewater influent concentrations during the study period were 307.5 mg L-1, 170.8 mg L-1, 3.1 mg L-1, and 0.015 mg L-1 for BOD5, NH4+, NO3-, and NO2-, respectively. Mean effluent concentrations leaving the fourth wetland bed were ND, 1.9 mg L-1, 4.2 mg L-1, and .002 mg L-1, for CBOD5, NH4+, NO3-, and NO2-, respectively. Eh values at the 15-cm-depth were highest in the first wetland bed with a mean Eh value of 440 60 mV (maximum value of 610 mV, minimum value 274 mV). The other 3 beds had similar maximum Eh values 549, 555, and 529 mV, respectively. Minimum values in these three beds, however, were significantly different from the first bed (76, 23, and 48 mV, respectively).