A regenerative thermal oxidizer is provided with a combustion chamber and a plurality of heat transfer columns. Heat transfer media within the columns include solid surfaces which define vapor flow passages. To permit higher volumetric vapor flow rates nearer the combustion chamber, the passages are larger in cross-sectional area and the solid surfaces have less total surface area in those portions of the heat transfer media nearer the combustion chamber than in those portions away from the combustion chamber. The larger cross-sectional area of the passages and reduced area of the solid surfaces decreases the resistance to vapor flow and allows the passages to accommodate the higher volumetric flow rate of the vapor stream passing through the hotter portions of the heat transfer media. The relatively smaller cross-sectional area of the passages and increased area of the solid surfaces in that portion of the heat transfer media located away from the combustion chamber allows for more efficient heat transfer between the media and the vapor stream in those cooler portions of the heat transfer media.