The binary and ternary complex stability constants between 2,3-dihydroxybenzoic acid (DA) and N-acetylcysteine (Nac) with the divalent metal ions (M) Mn2+, Co2+, Ni2+, Cu2+ and Zn2+were studied in aqueous solution at 310.15 K and an ionic medium of 0.15 mol·dm−3 NaCl. The complexes’ stability constants (log10 β), refined from the potentiometric data using the Hyperquad2008 program, indicate that the ternary complexes are more stable than the binary complexes. The stability constants were supported by additional computation, refined from the spectrophotometric data using the Hypspec program. The values of the ternary complex stability relative to their binary complex (Δlog10 K) and the disproportionation constant (log10 X) indicate that formation of ternary complex species [M(DA)(Nac)]3− is more favorable than that of species formed by two identical ligands, [M(DA)2]4− or [M(Nac)2]2−. For the investigated M, the stability of complexes follows the trend Cu2+ > Zn2+ > Ni2+ > Co2+ > Mn2+.