Oxygen microenvironments adjacent to mussel (Mytilus edulis) and periwinkle (Littorina littorea) tissues were described using micro-optodes. For mussels these environments did not differ from the surrounding water body in either static or stirred conditions. Consequently no difference was seen in the MO2 of mussels within stirred and unstirred respirometers. An oxygen extraction efficiency of 30% was recorded for mussels in stirred and unstirred conditions. Under static conditions, periwinkles, with their weak ventilatory arrangements, were not capable of ventilating the surrounding water efficiently, and their tissues were surrounded by hypoxic seawater, even in fully aerated water. The resultant build up of oxygen gradients close to the tissue surface led to measures of oxygen consumption representative of hypoxic conditions in fully aerated, unstirred respirometers. Stirring of the contents of the respirometer immediately prior to final oxygen concentration measures was insufficient to counter this effect, which could only be avoided by continuous artificial stirring of the respirometric chamber water. These findings have considerable implications for study of the oxygen consumption of aquatic animals with limited ventilatory capacity, such as gastropod molluscs.