The invention relates to a continuous carbonylation process for high turnover carbonylation, and a carbonylation reaction medium and product stream thereof. The process comprises carbonylating an ethylenically unsaturated compound with carbon monoxide in the presence of a source of hydroxyl groups and a catalyst system. The catalyst system comprising: (a) a bidentate phosphine, arsine or stibine ligand; and (b) a catalytic metal selected from a group VIB or group VIIIB metal or a compound thereof. The catalytically active concentration of said catalytic metal, measured as the ACCF (product Kg. hr<-1>. Dm<-3>), is maintained at less than 0.5.