PURPOSE: To carry out catalytic carbonylation with no loss of the catalyst.

CONSTITUTION: An olefin, an alkanol, a carboxylic acid or ester is allowed to react with carbon monoxide in the presence of a group VIII metal-contg. catalyst. The resultant carbonylation product soln. is conveyed to a separation zone kept under a lower pressure than the reaction zone. During the conveyance, a carbon monoxide-contg. gaseous stream is introduced so that the carbon monoxide contributes a partial pressure of up to about ≤30 psia of the total pressure in the separation zone and a portion of the carbonylation product soln. is flashed and separated. The uniform catalyst existing in the liq. carbonylation product soln. remains in soln. form and is recycled back into the reaction zone together with the unflashed carbonylation product soln. The flashed and removed product soln. is conveyed to a purification zone and the objective product is separated from undesirable by-products.