To investigate the effects of carbon surface characteristics on NH 3 adsorption, coal-based and coconut shell activated carbons were modified by treatment with oxidants. The surface properties of the carbons were characterized by low temperature nitrogen sorption, by Boehm's titrations and by XPS techniques. NH 3 adsorption isotherms of the original and the modified carbons were determined. The results show that the carbons were oxidized by HNO 3 and (NH 4 ) 2 S 2 O 8 , and that there was an increase in oxygen containing functional groups on the surface. However, the pore-size distribution of the coal-based carbons was changed after KMnO 4 treatment. It was found that the NH 3 adsorption capacity of the modified carbons was enhanced and that the most pronounced enhancement results from (NH 4 ) 2 S 2 O 8 oxidation. Under our experimental conditions, the capacity is positively corrected to the number of surface functional groups containing oxygen, and to the number of micro-pores. Furthermore, an empirical model of the relationship between NH 3 adsorption and multiple factors on the carbon surface was fit using a complex regression method.