The use of water-soluble catalysts represents a significant advance in homogeneous: catalysis in general and in the manufacture of n-butyraldehyde in special. Although catalyzed homogeneously, the technique of an "immobilized catalyst in mobile phase" offers all advantages of a heterogeneous process. Thus, the combination of homogeneous catalysis with heterogeneous catalyst handling yields the most straightforward and soundest oxo process with superior economics.