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Establishing the features of acid-base and electrochemical behavior in “sulfur dioxide – 1,3,5-tris-(2-hydroxyethyl)hexahydrotriazine – water” solutions is undoubtedly an actual and important task. A pH-, redox-, and conductometric studies of protolytic equilibria were carried out for the solutions containing 1,3,5-tris-(2-hydroxyethyl)hexahydrotriazine (TZ; the reaction product of monoethanolamine with formaldehyde interaction), its protonated form (TZH+), N-(2-hydroxyethyl)aminomethanesulfonic acid (HEAMSA), and its anion, N-(2-hydroxyethyl)aminomethane-sulfonate (HEAMS) at a constant total content of amine nitrogen 0.1 mol/L with varying content of absorbed sulfur dioxide 0 £ QSO2 £ 0.12 mol/L at temperature range 273–313 K. It was shown that the addition of formaldehyde to solutions of “sulfur dioxide – monoethanolamine – water” leads to a decrease in their pH by 0.09–4.75 units, which is obviuosly caused by the formation of HEAMSA. The symbiotic nature of the change in DpH values with temperature (SO2 : N < 1.0 : 2.0) is noted at the same content of components in the solutions. Conductometry data indicate the association of free monoethanolamine, its ammonium sulfites and hydrosulfites into less mobile TZ, TZH+, HEАМSA and HEAMS–. Based on the developed mathematical model using pH-metry data, the ion-molecular component composition of “sulfur dioxide – 1,3,5-tris-(2-hydroxyethyl)hexahydrotriazine – water” solutions at 273–313 K was calculated. The concentration dependences on the ionic strength of the solutions were obtained. The concentration constants of TZ protonation and HEAMSA dissociation were estimated. It was shown that under the experimental conditions the thermodynamic dissociation constant of HEAMSA is practically independent on temperature and is equal to 10.14 ± 0.08. The obtained results are recommended to be used in the development of effective chemisorbents of sulfur dioxide.