Pyrolysis of isonitroso Meldrum's acid is probed to generate gaseous nitrosoketene at temperatures above 80 °C under vacuum in a static system by FTIR and mass spectroscopies. Previously, cyclic nitrones were synthesized from the isonitroso Meldrum's acid in refluxing toluene and ketones. This spectroscopic observation gives a support that cycloaddition of nitrosoketene and ketones gives the cyclic nitrones. Further evidence for the production of nitrosoketene is given by the comparison of vibrational frequencies between experimental results and ab initio calculations. At temperatures exceeding 160 °C, nitrosoketene undergoes decarbonylation, forming CO.