The characterisation of beryllium phosphinates of stoicheiometry Be(Bun2PO2)2, Be(Ph2PO2)2, and of some mixed-ligand species Be[(Bun2PO2)2 – 2x(Ph2PO2)2x] is reported. The compounds richer in alkyl groups give stable solutions in hydrocarbon and halogenated solvents. Solution properties indicate that these compounds are linear polymers which undergo a reversible degradation influenced by the nature of the solvent, concentration, and temperature. The mixed-ligand species can be crystallised and are formulated as random copolymers. Isomorphous replacement of the two kinds of phosphinate groups within the crystal lattice of the related homopolymers is observed. The X-ray diffraction fibre photographs indicate that the homopolymer [Be(Bun2PO2)2]n and the copolymers {Be[(Ben2PO2)2 – 2x(Ph2PO2)2x]}n(x= 0·20 or 0·33) have a backbone structure with alternate single and triple bridging phosphinate groups between tetrahedral metal atoms. The same chain-type structure is proposed for the homopolymer [Be(Ph2PO2)2]n and the copolymers richer in diphenylphosphinate groups.