Sediment trap particles, as well as particulate organic material including particles larger than 10 pm collected by in situ pumps (ISP) and fresh corpses of the gelatinous zooplankton species Thalia democratica, were collected in the northwestern Mediterranean Sea from April to July 1995, and incubated (after mixing with 0.2 pm filtered seawater) under laboratory conditions with their own bacterial assemblages for 6 to 24 d in batches under oxic conditions and in the dark. Particulate (POC > 0.7 mu m), dissolved (DOC < 0.7 mu m) and colloidal (0.02 < COC < 0.7 mu m) organic carbon contents, as well as bacterial abundance and production, were quantified over time. In all experiments, total organic carbon (TOC = POC + DOG) decrease covaried with an increase in bacterial abundance and production, bacteria being the main mediators of particle decomposition. We found that COC accounted for 19 to 31% of DOC immediately after particle dilution in 0.2 pm filtered seawater, and always for less than 9 % at the end of the experiments. As organic colloids comprised less than 7 % of DOC in the 0.2 pm filtered seawater used to dilute the particles, this result suggests that COC was mainly produced from particle decomposition. Assuming that bacterial populations were the sole decomposer of organic matter in the batches, the results gave bacterial growth efficiencies (BGE) in the range of 3 to 21%, indicating that decomposition of these particles significantly produce CO2 through bacterial respiration. The results showed that bacteria degraded 16 to 87% of the initial amount of POC within the first 48 h, whereas only 6 to 22 % of POC was degraded in the second stages. Our data and modeling work based on such short incubation times suggest that salp bodies are composed of 1 labile and 1 refractory organic fraction, whereas both ISP- and trap particles are composed of 2 labile and 1 refractory organic fraction. A 1G-model (for salp) and a 2G-model for other particles was able to satisfactorily reproduce the data sets.