Achieving the desired selective transformations of the very stable CO2 into useful chemicals is quite important for the development of economically and environmentally sustainable synthetic methods. Herein, mass spectrometric experiments and quantum-chemical calculations have identified that ScNH+ reacts quite efficiently with CO2 under thermal collision conditions to exclusively yield ScO+ and isocyanic acid (HNCO). This is a novel reaction type in CO2 activation reactions mediated by gas-phase ions. In this reaction, the C=N double bond has also been formed for the first time in the gas phase. The mechanism of "migratory insertion" is proposed. Coupled with the previously reported reaction of Sc+ with NH3, HNCO can be synthesized under mild conditions from NH3 and CO2 in quite simple reactions. The mechanistic information gained in this gas-phase model reaction can offer fundamental insights relevant to corresponding processes and further guide on how to design brand new catalysts.