The effects of temperature on intracellular pH were investigated in the abdominal muscle tissue of two latitudinally separated populations of the euryhaline and eurythermic common sand shrimp Crangon crangon and in the stenohaline and stenothermic deep water shrimp Pandalus borealis. The contribution of passive mechanisms (due to the physico-chemical responses of intracellular buffers) and active mechanisms (due to ion exchange) to the pH change was quantified at different temperatures. In addition the extracellular ion composition was measured at the same temperatures. Acclimation in full strength sea water at various temperatures had relatively minor and inconsistent effects on haemolymph ion concentrations. The changes in pHi due to temperature were not reflected by alterations of haemolymph ion concentrations. The pHi/T-relationship after 4 h of incubation at various temperatures differs between the two populations of C. crangon, with ΔpHi/Δ°C values of -0.008 in North Sea C. crangon and -0.018 in C. crangon from the White Sea while the absolute pHi values in White Sea C. crangon were lower by about 0.15 pH units than in North Sea C. crangon or in P. borealis at the same temperature. In P. borealis as in White Sea C. crangon, 4 h of incubation were sufficient to regulate ΔpHi/Δ°C at a level (-0.016) close to that predicted by the alphastat hypothesis (ΔpHi/Δ°C = -0.018, Reeves, 1972). For the two populations of C. crangon the contribution of active mechanisms to alphastat control was about 50% compared to only 15% in P. borealis. In winter and summer animals of North Sea C. crangon the passive fraction was 0.009 ΔpHi/Δ°C and thus similar to those observed after 4 h of incubation (-0.008 ΔpHi/Δ°C), indicating the absence of active mechanisms during this period. The subsequent increase in ΔpHi/Δ°C after 6 days of incubation (-0.017 units · °C-1) demonstrated that the active adjustment of constant alpha imidazole was slower in the boreal North Sea C. crangon compared to Arctic P. borealis, while in the sub Arctic White Sea C. crangon time-dependent changes in the pHi/T relationship resulted in constant alpha imidazole after 12 h of incubation with a value only slightly larger than the one seen after 4 h.