Fly ash is the residue from the combustion of pulverised coal in thermal power stations. It consists of a variety of particle sizes of hollow silica-alumina spheres, which can be utilised as a custom made filler material in a number of industries, e.g. plastics, rubber, paint, refractories, etc. The 5 mu m average particle size fraction is popular for its application in plastic products. In order to comply with the requirements for application in plastics, it is essential to understand and characterise the surface of the ultrafine glass spheres coming into contact and interacting with the organic phase of the final composite. This contribution describes the use of a number of techniques, such as EPMA (Electron probe micro-analysis), Raman spectrometry, zeta potential measurements and ICP-OES (Inductively coupled plasma optical emission spectroscopy) to investigate the surface composition and behaviour of the ultrafine fly ash particles. (c) 2005 Elsevier Ltd. All rights reserved.