Peat is a hetorogenous material containing humic substances (HS) that increase its fertilizer value, and acid functional groups that react with ammonia. However, polyvalent cations may block some ammonia-sorbing sites. Sixty-seven peat samples were collected in Quebec (i) to relate the colorimetric Kaila pyrophosphate test for extracting HS to the von Post field test and (ii) to correlate ammonia sorption capacity (ASC) to selected peat properties as follows: von Post test, Kaila pyrophosphate test, pH(H2O), pH(CaCl2), pH(KCl), exchangeable acidity as well as ash, Ca, Al and Fe contents. Exchangeable acidity was the most highly correlated with ASC (P < 0.01). Using field-moist material and a 1:4 peat:solution volumetric ratio, the most reliable criteria were pH(CaCl2) < 4.4 and pH(KCl) < 4.5. Using air-dry peat and a 3:50 peat:solution ratio, the corresponding criteria were pH(CaCl2) < 4.3 and pH(KCl) < 4.1. The pH(CaCl2) method provided the most consistent determination of peat acidity, whether taken on a volume or weight basis. A minimum pyrophosphatate test value between 0.1 and 0.2 absorbance unit is apparently required for making peat composts and peat-ammonia-mineral fertilizers enriched in soluble humic substances. The pH(CaCl2) method and the Kaila pyrophosphate test should be combined to routinely control the quality of peat materials for making peat composts and peat-ammonia-mineral fertilizers. Key words: pH, exchangeable acidity, pyrophosphate test, ammonia sorption capacity