A rhodium-calix[6]arene diphosphite complex was used as a catalyst in the hydroformylation reaction of linear alkenes and styrenes. The influence of P/Rh and substrate/catalyst ratios, as well as temperature and pressure, on the aldehyde yield and the regioselectivity, of the process was studied. The complexation of calix[6]arene diphosphite with Rh(acac)(CO)(2) was studied by H-1 and P-31 NMR spectroscopy and atmospheric-pressure electrospray ionization (AP-ESI) mass spectrometry.