The complex cis-[Rhl(CO)(Ph(2)PCH(2)P(S)Ph(2))] is eight times more active than the classic Monsanto catalyst [Rhl(2)(CO)(2)](-) for the carbonylation of methanol at 185 degrees C offering the first promoted system effective under conditions which allow industrially realistic rates; the X-ray crystal structure of the analogue cis-[RhCl(CO)(Ph(2)PCH(2)P(S)Ph(2))] is reported together with in situ spectroscopic evidence for species in the catalytic cycle.