A process for the polymerisation and copolymerisation of olefins is disclosed, comprising contacting the monomeric olefin under polymerisation conditions with a polymerisation catalyst or catalyst system which comprises (a) a source of a Group VIII metal; (b) a bidentate phosphine ligand having the formula (R1)(R1)P-X-P(R1)(R1), where each R1 is independently selected from a phenyl group or a substitued phenyl group with the proviso that at least one of the R1 groups is a phenyl group having at least one ortho substituent, and X is a bridging group of the structure- [N]¿x?-[P]y-[N]- where x and y are independently 0 or 1, or -C(R?4)¿2- where R4 may be the same or different and is hydrogen or a monovalent hydrocarbyl, substituted hydrocarbyl or hetero-hydrocarbyl group; and optionally (c) a promoter.