Sediments from the surface, core waterwaand vegetation of tropical mangroves were collected from tropical mangroves, India. Total phosphorus, total nitrogen, organic carbon, protein, carbohydrate and tannin and lignin in these samples were estimated. The C/N and C/P of the leaves present in Mangalavanam, Vypeen, and Nettoor are in good agreement with that of the sediment and this -result indicated that the main source of the sediment organic matter is the plant remains. The profile of C/N ratios shows a decrease towards the depth, but lies within that of the oligotrophic lakes and plant tissues and peat. The decay rate of C and N indicated that the main reactivity is in the first 5cm of the sediment, which is subjected to re-suspension and significant bio-turbation. But there existed a uniform decay pattern at 10 cm and 15 cm. In the case of monsoon season and post monsoon season a random character was observed at 15 cm depth, which can be attributed to the effective release of organic matter from that depth zone. To assess the reactivity of the systems, the classic one G model with slight modifications to suit to the system was employed.