Alkali-metal and calcium(II) complexes of monocarboxylate A^–(acetate and salicylate), dicarboxylate A^2–(malonate, maleate, succinate, malate, tartrate, phthalate, and oxydiacetate), or amino acid HA (glycine or L-histidine) ligands have been studied potentiometrically, using a glasssaturated calomel electrode, at different temperatures and ionic strengths. The monocarboxylate ligands form [MA] and the dicarboxylates [MA] and [M(HA)] species (charges omitted) with both alkali metals and calcium. Glycine and L-histidine form [MA]⁺ and [M(HA)]²⁺{and [M(H₂A)]³⁺ for L-histidine} complexes with Ca²⁺, whilst alkali metals form only [M(HA)]⁺ with glycine. Some interesting regularities in the formation constants are pointed out. From the dependence on temperature of formation constants, values of ΔH^⊖ and ΔS^⊖ have been determined. The function log β=f(I) has been carefully studied in the range 0.02 ⩽I⩽ 1 mol dm^–3. The reliability of a new model for the dependence on ionic strength of formation constants (when dealing with weak complexes it is in practice impossible to use the constant ionic medium method) is widely discussed. Two methods of calculation are described and the more general method has been checked by simulated curves.