A process for the polymerization and copolymerization of olefins is disclosed, comprising contacting the monomeric olefin under polymerization conditions with a polymerization 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(R⁴)₂— where R⁴ may be the same or different and is hydrogen or a monovavlent hydrocarbyl, substituted hydrocarbyl or hetero-hydrocarbyl group; and optionally (c) a promoter.