The heterogeneously catalysed gas phase hydroformylation of ethylene to propionaldehyde was studied over solid RhCl(PPh 3 ) 3 and RhCl(CO)(PPh 3 ) 2 . At 3 bar and 185°C, an active phase formed from RhCl(PPh 3 ) 3 which was different from RhCl(CO)(PPh 3 ) 2 under the reaction conditions studied. The selectivity of RhCl(PPh 3 ) 3 to propanal was much better than that of supported Rhmetal. Thus, solid metal-complexes operated in gas phase reactions clearly hold promise as a new class of heterogeneous catalysts.