A process or the liquid phase carbonylation of methanol or a reactive derivative thereof comprises contacting carbon monoxide with a liquid reaction composition comprising methanol or a reactive derivative thereof, a halogen promoter and a rhodium catalyst system comprising a rhodium component and a bidentate phosphorus-sulphur ligand, the ligand comprising a phosphorus dative center linked to a sulphur dative or anionic center by a substantially unreactive backbone structure comprising two connecting carbon atoms or a connecting carbon and a connecting phosphorus atom. Novel rhodium ligand complexes of formula [Rh(CO)L].sub.m and Rh(CO)LY] wherein Y is a halogen, m is a number less than 10 and L is ##STR1## wherein the R groups are independently selected from C.sub.1 to C.sub.20 alkyl, cycloalkyl, aryl, substituted aryl and optionally substituted aralkyl; the R.sup.5 groups are selected from hydrogen, C.sub.1 to C.sub.20 alkyl, cycloalkyl, aryl, substituted aryl, aryl and optionally substituted aralkyl groups; the R.sup.1 to R.sup.4 groups are independently selected from hydrogen, C.sub.1 to C.sub.20 alkyl and optionally substituted aralkyl; X is a non-coordinating substitutent such as C.sub.1 to C.sub.20 alkyl, cycloalkyl, aryl, substituted aryl, C.sub.1 to C.sub.20 alkoxy, C.sub.1 to C.sub.20 alkylthio, trialkylsilyl or triarylsilyl, preferably, Si(CH.sub.3).sub.3 or methyl and n is 0 to 4 preferably 0 or 1.