Experiments demonstrated that the effect of water activity on the solvate complex of virions underlies the mechanism of thermal inactivation of the foot and mouth disease virus in suspension; changes in the structure of the complex impair the electrophysical balance of the RNA-protein relations and hence, destroy the virus. Heating of virus-containing suspensions at moderate positive temperatures leads to destruction of the noninfectious part of virus population of 146S particles, thus decreasing the infectious activity of the virus.