Extended Hildebrand Solubility Approach (EHSA) was applied in this research to analyse the equilibrium solubility of sulphadiazine in some {N-methyl-2-pyrrolidone (1) + water (2)} and {Carbitol® (1) + water (2)} mixtures at 313.15 K. Reported experimental solubilities and melting properties of this drug were used for EHSA calculations. Good predictive character of EHSA (with mean deviations lower than 1.7%) was found by using regular polynomials in order five by correlating the interaction parameter (W) and the Hildebrand solubility parameter of solvent mixtures free of drug (δ1+2). However, the predictive character of EHSA was almost the same as obtained when logarithmic drug solubilities (log x3) were correlated with δ1+2 by using a fifth-degree regular polynomial.