Precipitation pH at a Russin EANET site, Irkutsk, was discussed and explained from the viewpoint of acid–base chemical equilibrium theory. The datasets employed were the wet-only daily basis observations at a subarctic climate urban site from 1999 to 2020. A distinct seasonal variation in pH was discovered for the monthly means over the entire period, from pH5.24 in July to pH6.66 in February, bracketing pH5.6, a criterion for acid rain. Monthly concentration variations in major ions, nss-SO₄²⁻, NO₃⁻, NH₄⁺, and nss-Ca²⁺, divulged that nss-Ca²⁺ and nss-SO₄²⁻ were predominant species and that nss-Ca²⁺ dominated nss-SO₄²⁻ during winter months and vice versa in summer. Individual pHs were explored with the concentration difference between two ions, ([nss-SO₄²⁻] − [nss-Ca²⁺]), defined as D₂, as well as two other differences in selected ion groups, D₄ = ([nss-SO₄²⁻] + [NO₃⁻]) − ([NH₄⁺] + [nss-Ca²⁺]) and D₆ = ([nss-SO₄²⁻] + [NO₃⁻]) − ([NH₄⁺] + [nss-Ca²⁺] + [nss-Mg²⁺] + [nss-K⁺]), to show some clear relationships with pH. The observed results were compared with theoretical calculations, D_n = [H⁺] − HP_CO2K_a1/[H⁺], where n is 2, 4, or 6. The distribution of ΔpH (=pH_calc − pH_obs) for D₂ demonstrated the most reasonable pattern with a median of zero among the three D_ns. These quantitative results conclude that the Irkutsk pH was controlled dominantly by sulfuric acid and calcium carbonate.