The short-term decomposition dynamics of Ulva rigida C. Agardh by a natural bacterial consortium were studied in the laboratory using a 14C radio-tracer technique. Following an initial rapid loss of approximately 15% of the particulate organic carbon (POC) by leaching of dissolved organic carbon (DOC), there was an exponential loss of POC and an accompanying increase in the labelling of the inorganic carbon (IC) pool. However, losses of labelled POC were significantly greater than the summed accumulation of radiolabel in the IC and DOC pools over the course of the experiments. We propose that this discrepancy may be due to the production of volatile organic compounds (VOC) during decomposition, which were stripped from the medium during the extraction of IC, by acidification and sparging with helium, but were not retained by the KOH traps for IC. The compatible solute dimethylsulphonioproprionate (DMSP) which is accumulated to high concentrations by Ulva spp. and is metabolised by bacteria to volatile dimethylsulphide (DMS) and potentially methane, could be one of the major sources of VOCs during our incubations.