A novel rearrangement of the omega-phenylalkanals phenylacetaldehyde, 3-phenylpropionaldehyde and 4-phenylbutyraldehyde towards phenyl alkyl ketones is reported. The liquid-phase isomerizations are catalyzed by cesium oxide or binary cesium-lanthanum oxide supported on the mesoporous aluminosilicate molecular sieves MCM-41 and HMS and on amorphous supports like silica-alumina and gamma-alumina. Beside the title reaction the aldol condensation is observed as a side-reaction. A mechanism is proposed in which the aromatic ring is involved in the rearrangement process. Factors governing the activities and product selectivities are presented.