Batch catalytic and high pressure infra-red studies of catalytic systems have provided detailed information about the homogeneous rhodium-chloride catalysed hydrocarbonylation of ethene to form propanoic acid in CH(_3)COOH/c.HC1 at 180ºC and 60 bar pressure. [Rh(CO)(_2)Cl(_2)]- was shown to be an active catalyst for this process. Tin(ll) chloride as a co-catalyst was shown to have an effect on the rate of propanoic acid production for a rhodium-chloride catalytic process, but a clear promotional effect was only observed for systems employing a Sn:Rh molar ratio of 2: 1. Sn:Rh molar ratios higher than 2: 1 lead to a decrease in the selectivity for propanoic acid, while those lower than 2:1 result in catalytic activity consistent with [Rh(CO)(_2)Cl(_2)]- being an active species. The reactions of rhodium(I) carbonyl chlorides with tin(II) chlorides were carried out at atmospheric pressure to investigate the reaction chemistry between species present in the catalytic process. [Rh(CO)(_2)Cl(_2)]- and Rh(_2)(CO)(_4)Cl(_2) react with SnCl(_2) and SnCl(_3)- in both THF and CH(_2)Cl(_2) to form Rh(I)-CO-SnCl(_3) complexes. Infra-red and (^119)Sn NMR data identified the 5-coordinate complex [Rh(CO)(_2)(SnCl(_3)) (_3)](^2-)- as the favoured species formed in solution. Its crystal structure is reported. However, it appears to be related by a series of facile reactions, involving dissociation of SnCl(_3)(^-)- and CO groups, to several other 4 and 5-coordinate Rh(I)-CO-SnCl(-3) complexes. [Rh(CO)(SnCl(_3))(_4)](^3-), [Rh(CO)(SnCl(_3))(_2)Cl](^2-), and [Rh(CO)(_2)(SnCl(_3))X](^-) (X = Cl(^-) or SnCl(_3)(^-)) have been isolated from solution, often as mixtures along with [Rh(CO)(_2)(SnCl(_3))(_3)](^2-). Spectroscopic and X-ray crystallographic data indicated that SnCl(_3)(^-)- is a significant π- tacceptor ligand, thus explaining its ability to form 5-coordinate 18-electron rhodium(I) complexes. The effect of tin(II) chloride on the rhodium-chloride catalytic reaction is attributed to changes in π-acceptor and trans effect properties when SnC1(_3)(^-)- replaces chloride ligand(s). High pressure infra-red studies are consistent with both [Rh(CO)(_2)Cl(_2)](^-) and a Rh(I)CO- SnCl(_3) complex being catalytically active in a rhodium-tin-chloride system, and the carbonyl absorptions observed are consistent with [Rh(CO)(SnCl(_3))(_4)](^3-), [Rh(C0h(SnC(_3))(_3)](^2-) and [Rh(CO)(SnCl(_3))(_2)Cl](^2-) as catalytic precursors. The general predominance of [Rh(CO)(_2)(SnC1(_3))(_3)](^2-) in solution at atmospheric pressure and room temperature, suggests that it may be the favoured catalytic precursor, undergoing conversion to a catalytically active 16-electron Rh(I)-SnC1(_3) complex such as [Rh(CO)(_2)(SnCl(_3))X]- (X = Cl(^-) SnCl(_3)(^-)) via dissociation of SnCl(_3)-.