In ethanol,[RhX (CO)(PEt 3) 2] added directly or formed in situ from [Rh 2 (OAc) 4]· 2MeOH (OAc= O 2 CMe) and PEt 3 or [Rh (OAc)(CO)(PEt 3) 2] catalysed the carbonylation of CH 2 CHCH 2 X (X= Cl, Br or I) to ethyl but-3-enoate with CH 2 CHCH 2 OEt as a side product. Small amounts of the isomerisation product, ethyl but-2-enoate were produced but no base was required for the reaction. The selectivity of the reaction is in the order Cl> Br> I and ...