Established hypothesis, derived predominantly from studies on wholly-aquatic polar organisms, indicates that the upper thermal limits of aquatic animals are defined by oxygen limitation. In this study Littorina littorea and Nucella lapillus, eurythermal temperate intertidal gastropods of different habit and life history, were investigated to test the generality of this hypothesis. Two separate hypotheses were tested on both species. Hypothesis 1 was that thermal tolerance in water will be increased under hyperoxic conditions and decreased under hypoxic conditions. Median upper lethal temperature/s (MULT) were established under normoxic (21 % O-2), hypoxic (10 % O-2) and hyperoxic (40 % O-2) conditions in seawater. For Nucella lapillus the hypothesis was supported, since MULT increased with increasing available oxygen in seawater (4.6 degrees C difference between hypoxic and hyperoxic conditions). For L. littorea the hypothesis failed, since MULT did not differ significantly between different oxic conditions. Hypothesis 2 was that thermal tolerance in water-saturated air (normoxia) would be higher than in normoxic water due to the greater availability of oxygen in air. MULT was established for both species in water-saturated air (it had already been established in normoxic seawater). For L. littorea this hypothesis was supported by data, as MULT was 2.4 degrees C higher in air than in normoxic seawater. However, the hypothesis was rejected for N. lapillus, in which MULT in air was depressed by 1.6 degrees C by comparison with MULT in normoxic seawater. The departures from oxygen limitation theory can be explained by the species' habits, life history and respiratory capabilities in air and water..