1. In order to predict which species can successfully cope with global warming and how other environmental stressors modulate their vulnerability to climate related environmental factors, an understanding of the ecophysiology underpinning thermal limits is essential for both conservation biology a nd invasion biology. 2. Heat tolerance and the extent to which heat tolerance differed with oxygen availability were examined for four native and four alien freshwater peracarid crustacean species, with differences in habitat use across species. Three hypotheses were tested: 1) Heat and lack of oxygen synergistically reduce survival of species; 2) Patterns in heat tolerance and the modulation thereof by oxygen differs between alien and native species, and between species with different habitat use; 3) small animals can better tolerate heat than large animals and this difference is more pronounced under hypoxia. 3. To assess heat tolerances under different oxygen levels, animal survival was monitored in experimental chambers in which the water temperature was ramped up (0.25 ˚C min-1). Heat tolerance (CTmax) was scored as the cessation of all pleopod movement, and heating trials were performed under hypoxia (5kPa Oxygen), normoxia (20 kPa) and hyperoxia (60 kPa). 4. Heat tolerance differed across species as did the extent by which heat tolerance was affected by oxygen conditions. Heat tolerant species, e.g., Asellus aquaticus and Crangonyx pseudogracilis, showed little response to oxygen conditions in their CTmax, whereas the CTmax of heat sensitive species, e.g., Dikerogammarus villosus and Gammarus fossarum, was more plastic, being increased by hyperoxia and reduced by hypoxia. 5. In contrast to other studies on crustaceans, alien species were not more heat tolerant than native species. Instead, differences in heat tolerance were best explained by habitat use, with species from standing waters being heat tolerant and species from running waters being heat sensitive. In addition, larger animals displayed lower CTmax, but only under hypoxia. An analysis of data available in the literature on metabolic responses of the study species to temperature and oxygen conditions suggests that oxygen conformers and species whose oxygen demand rapidly increases with temperature (low activation energy) may be more heat sensitive. 6. The alien species D. villosus appeared most susceptible to hypoxia and heat stress. This may explain why this species is very successful in colonizing new areas in littoral zones with rocky substrate which are well aerated due to continuous wave action generated by passing ships or prevailing winds. This species is less capable of spreading to other waters which are poorly oxygenated and where C. pseudogracilis is the more likely dominant alien species.