The invention relates to a process for the hydroformylation of propylene with carbon monoxide and hydrogen to selectively form iso-butanal with respect to n-butanal, the process implementing a hydroformylation catalyst in combination with a phosphine, the hydroformylation catalyst comprising a palladium catalyst which comprises the element PdX 2 , where X is a halogen element selected from iodine I and bromine Br, the phosphine is of the form the phosphine is of the form P(R)(R')(R"), where R, R' and R" each comprise an alkyl group or an aryl group or an alkoxy group or an amino group.