PURPOSE: To inexpensively obtain the title compounds in reduced production of aryl formate, high selectivity and high yield by carrying out a co-oxidation reaction of cyclic ketones and aromatic aldehyde in specific ranges of molar ratio of the cyclic ketones and aromatic aldehyde and throughput of the aromatic aldehyde in the co-oxidation method.

CONSTITUTION: When a cyclic ketone (e.g. cyclohexanone) and aromatic aldehyde (e.g. 2,4-dimethylbenzaldehyde) are co-oxidized with O2, a molar ratio of the cyclic ketone and aromatic aldehyde is kept in the range of 1.1:1 to 20:1, preferably 4:1 to 10:1 and throughput of the aromatic aldehyde per reaction liquid volume is kept in the range of 0.05-1.5mol/1.hr, preferably 0.15-0.8 mol/1.hr to simultaneously obtain lactones (e.g. -capro-lactone) and aromatic carboxylic acid (e.g. 2, 4-dimethylbenzoic acid). The method is industrially safe and lactones having excellent thermal stability are obtained.