PURPOSE: To obtain adipic acid or pentenoic acid by isomerizing or dehydrocarboxylating a compound which is commercially less desired.

CONSTITUTION: By mixing and reacting branched 6 carbon diacids, 5 carbon lactones, 6 carbon diacid dehydrides, acids such as branched 5 carbon monolefins-acids, and lactones in the presence of carbon monoxide the pressure of which is 0 to 2,000 absolute psi, an iodide cocatalyst and an iridium catalyst and heating it to a temperature of 175 to 230°C, the aimed compound is provided, wherein it is prefer that iridium content of the reaction mixture is 100 to 8,000 parts per 1,000,000 weight part.