The present invention concerns a process for preparing substituted arylhalophosphines and derivatives thereof. According to the invention a substituted haloaryl is lithiated to form a substituted lithiumaryl which is subjected to metal exchange to provide a metal exchanged organometal compound. After reaction with a halophosphine compound a substituted aryl halophosphine is obtained. The present invention also provides a novel process for hydroformylation of olefinic compounds. The hydroformylation is carried out in the presence of a catalyst system based on a rhodium precursor and heterodonor ligands of the formula (I): YX1X2X3, wherein Y is phosphorus or arsenic and X1, X2 and X3 are each independently selected from aromatic groups comprising substituted phenyl and pyridyl groups, the substituents being hydrocarbyl groups attached to the aromatic ring at ortho-position via a heteroatom. With the various ligands of the present invention, i/n-ratios in range of 5 to 30 are obtainable. In the case of hydroformylation of methyl methacrylate, the selectivity of the methyl α-formylisobutyrate is 80 - 90 % and the amount of byproducts, such as methyl isobutyrate, is small.