A time-of-flight secondary ion mass spectrometry (TOF-SIMS) with a feature of in situ micro-cross-sectioning of samples was applied to coal fly ash particles. The apparatus is equipped with two gallium focused ion beams (Ga-FIBs); one is used for micro-cross-sectioning of a particle to expose an analyzing area at aimed part, and the other is pulsed for TOF-SIMS analysis. By using this apparatus coal fly ash particle surfaces and interiors were analyzed. Hollow and solid particles were found and their internal structures were revealed with secondary electron imaging and mass spectrometric measurements. The main constituents were Al and Si with minor ones of Na, Mg, K, Ti and Fe. As for the hollow particle, it was found a difference in both inorganic and organic constituents between its outer and inner surface. On the surface, Na, Mg, Ca, Cl, and also organic compounds were enriched. Solid particle had a uniform inorganic composition over its surface and interior. [DOI: 10.1380/ejssnt.2004.45]