An investigation has been made of the properties of a catalytic system based on rhodium hydrocarbonyl modified with tri(p-nonylphenyl)phosphite for the hydroformylation of olefins. It has been found that, under certain conditions, the catalytic system may be deactivated. By means of IR and NMR spectroscopy it has been established that deactivation of the catalyst is caused by its demodification as a result of hydrolysis and oxidation of the ligand with the formation of phosphorus compounds that are coordination-indifferent to rhodium.