Two processes are described for improving reaction rates for relatively hydrophobic substrates in aqueous biphasic systems. In the first, 1-octyl-3-methylimidazolium bromide ([Octmim]Br) increases the rate of hydroformylation of 1-octene from 8% conversion in 24 h to full conversion of 1.5 h. Phase separation is fast and catalyst retention is good. 1-Hexyl-3-methylimidazolium bromide gives little rate enhancement, whilst 1-decyl-3-methylimidazolium bromide gives stable emulsions., The mechanism of action of these additives is discussed. In the second approach, functionalising PPh 3 with amidine groups allows the rhodium catalysed hydroformylation of 1-octene in toluene with a very high reaction rate. The catalyst can be switched between toluene and water by bubbling CO 2 and back into toluene by bubbling N 2 at 60°C. This switching has been used to separate the catalyst from hydrophobic (from 1-octene) or hydrophilic (from allyl alcohol) aldehydes obtained from hydroformylation reactions. CO 2 expanded liquids have been shown to be effective media for transporting substrates and catalysts over supported ionic liquid phase (SILP) catalysts. The advantages offered over all gas phase and liquid phase catalysts are discussed.