Abstract. AIM: The aim of this study was to evaluate the initial stage of decomposition of two aquatic macrophytes (Typha domingensis and Eleocharis interstincta) under aerobic and anaerobic conditions; METHODS: The samples of aquatic macrophytes and water were collected in Cabiúnas lagoon, Rio de Janeiro, Brazil. In the experiment 1 mineralization chambers were prepared with fresh plant fragments and water from the lagoon in aerobic and anaerobic conditions. DOC and CO2 formation and O2 consumption were measured during 30 days. In the experiment 2 mineralization chambers were prepared with aquatic macrophytes leachate and glucose for each experimental condition. CO2 formation and O2 consumption were measured during 1 day; RESULTS: In experiment 1 DOC concentrations in T. domingensis decomposition were higher. Despite this, CO2 formation was more efficient in E. interstincta decomposition. The oxygen consumption increased in the beginning of decomposition and tended to stabilize towards the end. The O/C values increased in the beginning of decomposition and decreased thereafter. In experiment 2, aerobic decomposition presented higher CO2 formation. E. interstincta leachate presented higher CO2 concentration than T. domingensis; CONCLUSION: Our results showed that aerobic decomposition in the initial stages is higher only in T. domingensis mineralization. Oxygen consumption varies according to the alterations in the chemical composition of the detritus. The formation of CO2 from T. domingensis and E. interstincta decomposition in the water column is irrelevant in Cabiúnas Lagoon. The low DOC release from fresh aquatic macrophytes detritus indicates that particulate organic matter mineralization is the main route of T. domingensis and E. interstincta decomposition.