The effects of oxygen transfer on carbon burning rates measured in particle beds with a DTG are considered. Transfer both to the surface of the particulate bed and within the bed is analysed under typical conditions. Oxygen supply to the bed surface is depleted generally under those conditions (high bed Thiele modulus) when there are oxygen transfer limitations within the bed. The value of the bed Thiele modulus also determines whether combustion takes place throughout the bed or in a surface layer.