Although carbon monoxide (CO) is considered a practical source of the carbonyl functionality in various compounds, handling CO gas is difficult. The phase-vanishing (PV) method, using highly fluorinated solvents as the phase screen, was thus employed, in which CO was evolved for use in organic synthesis. An H-shaped reactor bearing two reaction chambers was employed. In the first chamber, CO was efficiently generated from sulfuric acid and ammonium formate under the PV conditions, and then consumed in the second chamber in a range of palladium-catalysed carbonylation reactions, affording the desired products. Use of this PV system allowed for easy and safe generation of hazardous CO gas, and its use thereof in organic synthesis.