Anaerobic tests with gravel from horizontal subsurface flow constructed wetlands (SSF) used for the treatment of urban wastewater were developed in order to evaluate the anaerobic biodegradability of their effluents. Two types of assays were conducted. The reactors used for the first type were glass vials of 45 mL, that were used for only one measurement, requiring starting experiments with a number of reactors equal to the measurements to be made. For the second type of experiments multiple measurements were done in the same reactors, by using flasks of 2.2 L. The COD of the SSF effluents used for the tests ranged from 60 to 130 mg/L. The evolution of CO2 in the headspace of the reactors was used as indicator of anaerobic biodegradation rates. CO2 mass emission rates ranged from 0.005 to 0.015 mu mol/mL day. CH4 generation was not detected in the tests in relation with the refractory properties of the effluent organic matter of the studied SSF. In situ measurement of CO2 and CH4 emissions from the gravel of the SSF ranged from 0.106 to 0.464 and from 0.039 to 0.107 mmol/m(2)h, respectively. Several CO2 fluxes measured in the field were quite consistent with the emissions observed in the laboratory. The developed tests can help to understand the performance of SSF and improve their operation. (c) 2006 Elsevier Ltd. All rights reserved.