Electrogenerated chemiluminescence (ECL) was used to image the spatial variations in electrochemical activity at the heavily doped polycrystalline diamond surface. ECL was generated by the reaction of [Ru(bpy)(3)](2+) and tripropylamine. Images of the chemiluminescence patterns at the polycrystalline diamond surface were recorded photographically after magnification with optical microscopy to show the location and size of individual active regions. The spatial distribution for ECL intensity indicated that the electrochemical reactivity at polycrystalline diamond electrodes was microscopically heterogeneous. The ECL intensities for (100)oriented growth sectors were much lower than those for other growth sectors, and remained at ca. 50% of those for (111) sectors even at the potential at which the intensity reached maximum. The ratios of the ECL intensities for the (100) sector to the average ECL intensities showed a linear relation with the potential, indicating that the conductivity for (100)-oriented microcrystallite is remarkably lower than that for other types of oriented microcrystallite. Micro-Raman imaging was used to investigate the microcrystallite-based heterogeneity for the conductivity at the heavily boron doped polycrystalline diamond. Raman spectra were collected from an area over 42 x 42 mum(2) including (100) and (111) growth sectors at intervals of 3 mum. The Zn map for the intensities of one phonon diamond line reveals that the regions of maximum line intensity correspond to (100)-oriented microcrystallite and the values are approximately 8 times higher than those at (111). The dependence of diamond line intensity on the boron doping levels in diamond indicates that the boron concentration in (100)-odented microcrystallite is I order of magnitude lower than that in (111). Heavily doped polycrystalline diamond film contains microcrystals with different boron doping levels, i.e., semiconductor and semimetallic diamond microcrystals. This microstructural heterogeneity for boron concentration might affect almost all electrochemical activity at heavily doped polycrystalline diamond electrodes.