CSP for the 1:1 adducts pyridine, pyridazine and pyrazine with formic acid at ambient pressure show that all of the them comply the ΔpKa rule and expectedly form co-crystals. However, under suitable pressures all of them form salts in violation of the ΔpKa rule. Specifically, at critical pressure, Pc = 3 GPa, 5 GPa and 15 GPa formic acid completely transfers its proton to pyridine, pyridazine and pyrazine forming pyridinium formate, pyridazinium formate and pyrazinium formate respectively. The additional electrostatic interactions gained for the ionic salt configurations under pressure overcome the proton-transfer energies. The present study reveals that pressure determines the eventual phase of aggregation for adducts of organic acids and bases. Therefore, apart the ΔpKa rule any attempt to predict the dynamics of the co-crystal ⇌ salt equilibrium should necessarily also consider pressure as a control factor. Existing crystals of adducts of organic acids and bases formed under pressure are in agreement with our predictions and additional experiments can fortify this hypothesis.