Formyl compounds represent one of synthetic integrants essential for the construction of complex organic molecules. In general, their activation at the carbonyl carbon by a Brønsted or Lewis acid has been used for the new bond formation at the carbon with a wide variety of nucleophiles. On the other hand, transition metal, especially late transition metal, complexes often activate a formyl C-H bond to generate an acylmetal hydride species (RC(=O)-M-H), which can react with a C-C unsaturated bond, leading to the hydroacylation. Furthermore, the C-H bond cleavage, followed by the migratory extrusion of the carbonyl moiety, generates a metal carbonyl species (M-CO) along with the release of R-H. This is recognized as the decarbonylation. The account describes a novel method for synthesis of carbonyl compounds, in which the decarbonylation process of formyl compounds is used as a carbonyl-donating process (CO gas-free carbonylation). It provides more reliable and accessible carbonylation reactions for synthetic organic chemists.