Ring-contracted main product formed by isomerization of isophorone oxide (1) is 2-formyl-2,4,4-trimethylcyclopentanone (2); a high yield of (2) could be achieved by use of zeolites as heterogeneous catalysts. The reaction could be carried out in a discontinuous liquid-phase batch reactor as well as in a continuous vapor-phase fixed-bed reactor. The formation of (4) by decarbonylation of (2) could be reduced by increasing the catalyst loading or by carrying out the reaction under short contact time in gas phase conditions.