The aim of this study was to investigate the chemical composition and cycling of dissolved organic matter (DOM), focusing on the influence of thermal stratification and mixing. Samples were collected at the surface, 500m and 1500m, in April, July and October 2004 at the DYFAMED time-series site in the Northwestern Mediterranean. High molecular weight (HMW) DOM was concentrated using ultrafiltration. The HMW DOM fraction was characterised by H NMR, amino acid and neutral sugar analysis. Results were interpreted in the context of the wealth of information available on site DYFAMED. Three key observations were made. Firstly, the carbohydrate component of DOM decreased with depth, in agreement with previous studies, indicating degradation of this labile material. Secondly, the July surface water sample was particularly carbohydrate rich; it is proposed this may be the result of increased carbohydrate production by phytoplankton under low nutrient conditions, and accumulation in the surface layers due to stratification of the water column. Finally, the October samples showed a distinctly different chemical signature to the April and July samples, potentially indicating a shift from a net production system in the spring and summer to a net re-mineralisation system in autumn. The results of this study offer an insight into the dynamic nature of DOM at station DYFAMED.