Four aryl-substituted secondary phosphane oxides (SPOs) were tested as preligands in the rhodium catalyzed hydroformylation of cyclohexene and 1-octene. Three of them form active hydroformylation catalysts through their phosphinous acid tautomers. n-Regioselectivities of up to 56 % were achieved in the reaction with the linear olefin. The catalytic system with the electron-poor SPO showed exceptional behavior. In situ high-pressure IR spectroscopic investigations accompanied by DFT calculations provide an explanation of the observed inhibition of the catalytic reaction. Furthermore, proof is given that noncoordinated SPOs readily react with product aldehydes at room temperature to form α-hydroxyphosphinic acids. In contrast Rh-catalyzed hydrophosphination did not take place.