An extremely acidified acid sulfate soil (ASS) was investigated to characterize its soluble and exchangeableacidity. The results showed that soluble acidity of a sample determined by titration with a KOH solutionwas much significantly greater than that indicated by pH measured using a PH meter, particularly for theextremely acidic soil samples. This is because the total soluble acidity of the extremely acidic soil sampleswas mainly composed of various soluble Al and Fe species, possibly in forms of Al sulfate complexes (e.g.,AISO4) and ferrous Fe (Fe2+). It is therefore suggested not to use pH alone as an indicator of soluble acidityin ASS, particularly for extremely acidic ASS. It is also likely that AISO4+ actively pericipated in cationexchange reactions. It appears that the possible involvement of this Al sulfate canon in the canon adsorptionhas significant effect on increasing the amount of acidity being adsorbed by the soils. An extremely acidified acid sulfate soil (ASS) was investigated to characterize its soluble and exchangeable acidity. The results showed that soluble acidity of a sample determined by titration with a KOH solution was much more significantly greater than than indicated by pH measured using a PH meter, particularly for theextremely acidic soil samples. This is because the total soluble acidity of the extremely acidic soil sampleswas mainly composed of various soluble Al and Fe species, possibly in forms of Al sulfate complexes (eg, AISO4) and ferrous Fe (Fe2 +). It is therefore suggests not to use pH alone as an indicator of soluble acidity in ASS, particularly for extremely acidic ASS. It is likely that also likely that AISO4 + actively involved in cation exchange reactions. It appears that the possible involvement of this Al sulfate canon in the canon adsorption has significant effect on increasing the amount of acidity being adsorbed by the soils.