ABSTRACT: The light and dark inundated fluxes of O₂ and total CO₂ (TCO₂), as well as the concentrations of chlorophyll a and phaeopigments, were measured (ex situ) on the upper and lower portions of 2intertidal mudflats ‹ 1 in the upper Huon Estuary (salinity 4 to 32) and 1 in a marine side-arm of the estuary (salinity 17 to 34) ‹ over 4 seasons. Dark-exposed fluxes of O₂ and CO₂ were also measured on the upper and lower mudflatsof both sites over 2 seasons. Exposed fluxes of O₂ were generally not significantly different to the fluxes measured during inundation. Exposed fluxes of CO₂ were generally 3 to 5 times lower than inundated fluxes ofTCO₂. At the more sheltered site in the upper estuary, significantly greater rates of primary production were measured on the upper mudflat. In contrast, the more marine site had lower rates of primary production, and no significant differencein rates of primary production were observed across the inundation gradient. It is proposed that a greater exposure to wave energy (as indicated by sediment grain size) at the marine site was the cause of the lower rates of primary production. Rates ofTCO₂ consumption in the light were generally greater than those of O₂ production. It is suggested that O₂ effluxes are greatly reduced in the light as a consequence of the re-oxidation of sulphides within the sediments.