A test system described by Van Straalen and Verhoef [J Appl Ecol 34 (1997) 217] was used in order to check whether the endogeic Collembolan Heteromurus nitidus was repelled by acid pH. In each of the eight experimental runs 16 naive animals were allowed to select sectors in a circular pH gradient made of pure quartz sand impregnated with McIlvaine's buffer solutions at constant osmolarity. Di-sodium or di-potassium hydrogen phosphate was mixed with citric acid in varying proportions, giving rise to acidity levels ranging from pH 2 to 9. The animals reacted quite differently according to whether Na or K was used as the metallic cation. With potassium, a strong variation was observed from one experimental box to another, H. nitidus aggregating at pH levels varying from 4 to 8, the most frequent aggregation being observed at pH 6. With sodium, aggregation occurred over a more restricted range, from pH 7 to 9, most frequent aggregation being at pH 8. It was concluded. that the most acidic pH range (2-3) was avoided by the animals and that the chemical composition of buffer solutions strongly influenced results of pH-preference tests. If we consider that (i) the absence of H. nitidus from acid soils (pH 5-3.5) can be explained by biological rather than by chemical effects, (ii) this species can be cultured in acid as well as alkaline soils in the absence of predators, it ensures that potassium gives results that are more representative of the distribution of H. nitidus according to soil acidity than sodium.