Subsurface flow constructed wetlands are extensive urban waste water systems commonly applied in small communities (< 2,000 PE). An intrinsic property of these wetlands is the low dissolved oxygen concentration (< 1 mg/l) usually found in their bulk water. Physical transfer at the air - water interface and transport mediated by plants are the main oxygenation processes in the wetlands. In this study, the total oxygen transfer rate (OTR) in a horizontal subsurface flow constructed wetland (HSFCW) was determined experimentally by using a tracer gas method. Conventional gas bottles (containing a mixture of gases with 87% propane) were used for the purposes of the study, because of low costs. The mass transfer ratio (relationship between oxygen increase and propane decrease) was measured specially for the purposes of the study. The ratio was determined over time in tap water placed in a tank of 20 L and found to be 0,99.