This research explores the effects of voids (pores on the particle surface that are deeper than their surface radius) on burning area at diffusion-limited combustion conditions. Scanning electron microscopy and digital processing of images of quenched particles were used to quantify surface Void area, perimeter, and reacting void wall area for voids with diameters larger than 1 mu m. After careful analysis, the most accurate determination of particle burning area at diffusion-limited conditions was achieved by measuring particle surface area using the technique of discrete revolution, subtracting surface void area, and adding reacting void wall area. In situ measurements of reacting coal particle temperatures and images were taken for three coals and spherocarb particles at conditions that limit the formation of CO2 from reacting carbon under various oxygen concentrations and heating rates. The results of these experiments indicate that correcting the measured surface area for void area and reacting void wall area produces calculated burning rates closely matching diffusion-limited burning rates for all conditions and all coals investigated. These results suggest that void area effects should be included for accurate determination of burning area at diffusion-limited conditions.