Application of electrospray ionization mass spectrometry (ESI-MS) to coordination chemistry has been extended in recent years. Here, we examined a number of polynuclear ruthenium(Ⅱ), rhodium(Ⅲ), and cobalt(Ⅲ) bipyridine complexes by ESI-MS. It was shown that ESI-MS is a useful tool for identifying metal complexes and detecting contamination, because ESI mass spectra for the complexes displayed a mass pattern simple enough for easy structural assignment. In coordination chemistry, the actual advantages of ESI-MS are as follows: (1) ionic metal complexes exhibit simple mass spectra that can be analyzed easily using the characteristic isotope distribution of transition metals, (2) the metal complexes yield multiply charged ions with loss of counter ions so that the multinuclearity of polymetallic complexes and self-assembled complexes in a solution can be determined, and (3) most importantly, since preformed ions in bulk solution are extracted to the gas phase in the soft ESI process, the observed mass spectra qualitatively reflect only the intact ions in the solution. We detected unstable species of Se or W complexes that exists only in very strongly basic or acidic solvents using the nanospray technique. Moreover, we also studied the application of chiral recognition using antimony potassium tartrate and the characterization of the self-assembly of ferrocenedicarboxylic acid in a solution by ESI-MS. The ESI technique combined with a flow-through reaction cell is a powerful tool for the detection of reaction intermediates and primary products. Finally, photosubstitution of Ru(Ⅱ) complexes, photo-induced metal release/inclusion of crowned malachite green leuconitrile derivatives, and electrolytic oxidations of Ru(Ⅱ) and Os(Ⅱ) complexes were investigated using online ESI-MS systems.%錯体の構造解析において，X線回折法やnuclear magneticrnresonance（NMR）分光法と比較して質量分析rn（mass spectrometry：MS）法から得られる分子情報は少rnないので，従来は，興味の対象となる錯体のX線回折法やrnNMR分光法による解析が困難な場合にのみ，質量分析法rnによる解析が行われていた．しかしながら，近年，錯体化rn学の分野で質量分析法がよく利用されるようになった．
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