Compositions of matter are disclosed which are highly useful in hydroformylation processes. The compositions are non-charged, non-chelated bis- and tris-(alkyl diaryl phosphine) rhodium carbonyl hydrides. The substituents of the alkyl group includes heteroorganic radicals containing ether, ester, keto and hydroxy oxygen, phosphine oxide and phosphorus ester phosphorus, amine, amide, amine oxide and heterocyclic nitrogen groups.These compositions are highly stable and selective catalysts for the hydroformylation of olefins under certain conditions. The disclosed catalyst systems contain a large excess of phosphine ligand and employ alpha-olefin plus synthesis gas reactant mixtures having a high H.sub.2 /CO ratio at relatively low pressures. They produce moslty aldehydes derived from terminal attack on the alpha-olefin reactant. In the case of low olefins, such as butene-1, the products can be advantageously removed in a continuous manner, as distillate components from a ligand reaction mixture kept at an elevated temperature and continuously fed by an appropriate mixture of the reactants.