The influence of a structural modification on thermodynamic aspects of solubility and hydration processes of 1,2,4-thiadiazole drug-like compounds was investigated. A substitution in the phenyl ring of the 1,2,4-thiadiazole molecule leads to a significant decrease of the solubility of these compounds. In order to rationalize the relationship between the structures of 1,2,4-thiadiazoles and their solubility, the latter was considered in terms of two fundamental processes: sublimation and hydration. It was found that for most of the compounds solubility decline is a result of a differently directed action of the sublimation and hydration contributions, i.e., the introduction of substituents leads to the simultaneous growth of the sublimation Gibbs energy and decrease in the hydration Gibbs energy.