Novel phosphinites and phosphonites of general formula: ##STR1## where R.sub.2 is an electron-donating substituent, R.sub.1 and R.sub.3 are electron-donating substituents or hydrogen atoms, provided that they are not both hydrogen atoms, or either R.sub.1 or R.sub.3 forms part of a fused alicyclic ring with R.sub.2, R is alkyl or cycloalkyl, and n and m are integers, each being either 1 or 2, provided that (m+n) equals 3, and when n is 2, the substituents on each aromatic ring may be the same or different, and their use as catalysts for the dimerisation of acrylonitrile to give predominantly straight-chain C.sub.6 dimers.