PURPOSE: To continuously obtain isobutyric acid in an economically advantageous equipment by communicating propylene, carbon monoxide and, if required, water and an alcohol into at least four reaction chambers in liq. hydrogen fluoride under pressure in high circulation mixing.

CONSTITUTION: Propylene and carbon monoxide of each one mole are continuously introduced into a reactor by using hydrogen fluoride of 5 to 15 times them as a Koch's catalyst under a pressure of 50 to 150 bar. The reactor is composed of four reaction chambers, where the reaction mixture is communicated to obtain the object. At this time, the primary reaction chamber is fed with a mixture composed of liq. and gaseous starting substance. The secondary reaction chamber is fed with the reaction mixture from the primary reaction chamber. The third reaction chamber is fed with the reaction chamber from the secondary reaction chamber, and the generated foams of the gaseous components are returned to the primary reaction chamber. The fourth reaction chamber is fed with the reaction mixture from the third reaction chamber, and the lower edge part is opened to the secondary reaction chamber. A coolant is flowed around the reaction chambers, and the constant reaction temp. is held.