The Oxford Scanning Proton Microprobe has been used to determine the elemental composition of individual fly ash particles in the 1-5 mu m geometrical size range, collected before and after a pilot electrostatic precipitator (3 MWe equivalent power) of a power plant burning South African coal (Amcoal). The electrostatic precipitator was working at high collection efficiency (99.8%) obtained by operating under semi-pulsed energization mode. Al, Si P, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga and Sr have been always detected in particles collected simultaneously both before and after the electrostatic precipitator. In some particles also V Ge, As and Zr resulted above the detection limits. Both at the ESP inlet and outlet one hundred particles have been analysed. According to major element concentrations, particles can be grouped into seven classes showing some preferential associations of minor and trace elements with specific matrix compositions. Most particles (about 80%) belong to classes 1 and 2 characterized by the same aluminosilicate nature with higher concentrations of several minor (P, Ca, Ti, Fe) and trace (S, Mn, Ni, Zn, Ga) elements in class 2 Within the same particle size range, the electrostatic precipitator seems to be selective with respect to particle composition: class 2 particles show a higher penetration. This result is discussed in terms of the particle composition effects on the fly ash electrical properties, which are relevant parameters for the electrostatic precipitation process.