Rates of organic carbon accumulation, mineralization and burial in sediments were examined during dry and wet seasons in four mangrove forests of a shallow-water embayment (Ao Sawi) in southern Thailand. Mass sediment accumulation (MAR), estimated from 210 Pb and 137 Cs pro®les, was rapid at all forests, ranging from 2.9 to 7.6 kg m 22 yr 21 ; mixed layer sediment thicknesses ranged from 16 to 38 cm. Total inputs of organic carbon ranged from 26.4 to 40.9 mol C m 22 yr 21 and burial rates ranged from 15.3 to 23.4 mol C m 22 yr 21 . Total rates of carbon mineralization, estimated from direct measurements of CO 2 gas ¯ux from exposed sediments, from DIC ¯ux from submerged sediments, and from core incubations for SCO 2 , ranged from 7.0 to 16.4 mol C m 22 yr 21 with few consistent seasonal differences among the four forests. Rates of O 2 gas ¯ux into exposed sediments ranged from 9.4 to 91.4 mmol O 2 m 22 d 21 and were signi®cantly greater than rates of dissolved O 2 ¯ux into submerged sediments (range: 21.9±38.3 mmol O 2 m 22 d 21 ). Oxic respiration was the largest decomposition pathway, with sulfate reduction (range: 0.6±16.9 mmol S m 22 d 21 ) the next most important pathway. Other metabolic pathways appeared to be minor, and methane was not detected in the porewater or in the gas ¯ux chambers. The discrepancy between rates of O 2 and CO 2 ¯uxes implies involvement in geochemical processes (e.g. sul®de oxidation, authigenic mineral formation). Sediment budgets indicate that organic carbon preservation was greatest (71% burial ef®ciency) in the oldest forest with equivalent burial ef®ciency (57±59%) in the younger forests. Mineralization ef®ciency ranged from 27±40% with the lowest ef®ciency at the oldest forest. These tropical mangrove forests are storage sites for sediment and, on average, retain approximately 60% of total input of organic carbon to the sediment.