The acidic properties of a Chernozemic humic acid were determined by equilibrium titration. The number of carboxyl groups is 2.828 (mmol/g) ± 0.5%. The results were used for an investigation of Brönsted acid catalysis of atrazine hydrolysis by a suspension of the humic acid. There is sufficient sorption so that the rates of change of atrazine and product hydroxyatrazine in solution differ both from each other and form the rate of atrazine conversion on the catalytic sites. An important objective of this work was the separate determination of the sorption equilibrium constants for atrazine and hydroxyatrazine, and the rate constant for atrazine conversion on the catalytic sites. All three constants are relevant to environmental research. The apparent rate constants for changes in solution are experimental artifacts however, not useful for such research. Calculated in terms of carboxyl sorption sites in the free acid form, the sorption equilibrium constants were 1.74 × 102, σ = 0.03 × 102 and 8.5 × 102, σ = 2.3 × 102, for atrazine and hydroxyatrazine, respectively. The rate constant for conversion on catalytic sites ranges from 1.8 × 10−1day−1 ± 20% to 2.9 × 10−1day−1 ± 20%, for atrazine coverage of sites from 9.0(μmol/g) to 1.0(μmol/g).