Surface effects of ozone treatment with a fluidized bed-type reactor on carbon black have been studied. It has been observed that the surface oxygen level is proportional to the total volume of ozone supplied per unit area of carbon black surface. The ozone treatment increases the acidity of the carbon black surface. A limiting pH value has been obtained at a surface oxygen concentration of around 6 at %. The surface oxygen introduced on the carbon black is most likely present as carboxylic acid groups as indicated by both high-resolution X-ray photoelectron spectroscopy and Fourier transform-infrared spectroscopy results. The ozone treatment of a carbon black decreases the tensile strength for a natural rubber (non-polar), but increases the tensile strength for a nitrile butadiene rubber (polar).