Deactivation of phosphinite catalysts (R(2) ’POR ") in the alcohol-promoted (R triple prime OH) linear dimerisation of acrylonitrile has been shown to proceed via two independent routes which may operate concurrently, each yielding the same cyanoethylphosphine oxide (R(2)’P(:O)CH2CH2CN) as the sole oxidation product. The first (main) route, the evolution of ethers, occurs with most alcohols. The second route, confined to tertiary alcohols, involves the evolution of alkene. The ethers evolved are mainly symmetrical, (R(2) triple prime O) and derived from the alcohol. Transesterification between the phosphinite and the alcohol component of the reaction mixture plays an important role in the overall reaction and leads to catalyst deactivation. The dimerisation reaction system most stable towards deactivation which gives a satisfactory selectivity to the linear product, is based on a mixture comprising acrylonitrile, toluene, isopropanol and isopropyl diarylphosphinites (Ar2POPri).