The objective of this investigation was to explain the variation in dielectric constants of mullite ceramics prepared with 25% excess silica under a variety of conditions as described in an earlier study. Different starting materials, mullite-silica (M-S) and alumina-silica (A-S), different particle sizes (fine or coarse) and different firing times and temperatures were used. Dielectric constants measured for the alumina-silica system at 1550°C at different times, showed an initial reduction followed by an increase on further heating. The dielectric constants of other systems increased with increase in heat-treatment time at the different heat-treatment temperatures. From the study of microstructures using transmission electron microscopy and energy dispersive X-ray spectroscopy, it was shown that the initial decrease in dielectric constant on heating the alumina-silica system was because alumina, a phase with a higher dielectric constant was being replaced by mullite, a phase with a lower dielectric constant. On completion of this reaction, the intergranular silica was absorbed by mullite thereby leading to an increase in the dielectric constant. It was then shown that whenever phases with a lower dielectric constant exist, the dielectric constant of the system as a whole is lower. © 1992 Chapman & Hall.