The existence of microsites of low pH around active colonies of nitrifying soil bacteria has previously been suggested but has been difficult to verify. A study was undertaken to examine whether observed decreases in bulk soil pH that occur during nitrification are in accordance with the theory of acidified nitrification microsites. A red earth soil (sieved <2 mm) was retained at a pH of 5·3 or amended with KHCO3 to achieve a pH of 6·3. Ammonium [(NH4)2SO4] was added to the soils and they were incubated for 35 days. In both soils the pH dropped rapidly and severely limited further nitrification. The soil with the higher initial pH experienced limitations to nitrification at a pH which was 0·2 units higher than that of the soil with the lower initial pH. The explanation for this result is in terms of acidified nitrification microsites. It is suggested that an active nitrifying colony may lower the pH within its immediate vicinity to a critical pH at which nitrification almost ceases. This critical pH achieved at the nitrification microsite is probably unrelated to the initial pH of the soil, but the pH of the soil matrix which is distant from the immediate influence of the nitrification microsite would remain at a pH closer to that of the soil initially. This less acidified region of the soil matrix would have an overriding influence on the measured pH of the bulk soil and account for the discrepancy between the measured pH of the two soils at the end of the incubation. These data provide further evidence that acidified nitrification microsites exist in soil, and that the measured soil pH is a poor estimate of the pH experienced by the microbial biomass.