Introduction Many analytical results obtained by electrospray ionization mass spectrometry (ESI-MS) have been reported since the applicability of ESI to biomolecules and polymers was presented by Fenn et al. in 1989 . Although characterization of labile organometallic compounds was also attempted by using ESI, molecular ions were not detected in most cases because of their instability. Even if the molecular ions are observed by using this ionization method, many fragment ions generated by thermal decomposition also appear in the mass spectrum. The heat given off from the desolvation chamber was thought to be necessary for ionization in the conventional ESI . However, we developed coldspray ionization MS, a variant of ESI-MS operating at low temperature, in order to detect labile organometallic compounds . In this report, we describe our new method, coldspray ionization mass spectrometry (CSIMS), which allows easy and precise characterization of labile organometallic compounds in solution.