In this study, surface functional groups on the oil-palm-shell adsorbents prepared by phosphoric acid (H3PO4) and potassium hydroxide (KOH) activation were detected using Fourier transform infrared (FTIR) spectroscopy. Oil-palm shell is an abundantly available carbonaceous by-product from palm-oil processing mills in some tropical countries like Malaysia, Indonesia and Thailand. Preparation of adsorbents from oil-palm shell is an economical and environmentally friendly utilization of these solid wastes. To develop more specialized and effective adsorbents from oil-palm shell, studies were carried out to investigate the effects of impregnation conditions on the textural and chemical characteristics of the prepared adsorbents. Adsorption and desorption tests of nitrogen dioxide (NO2) and ammonia (NH3) gases were used to evaluate the adsorptive capacities of the oil-palm-shell adsorbents. From the experimental results, it was found that surface functional groups, which were determined by the concentration of impregnation solution and the soaking time, had a significant influence on the adsorptive capacity due to the occurrence of chemisorption. Moreover, mechanisms of chemical activation of oil-palm shell using H3PO4 and KOH for the evolution of porosity and the formation of surface functional groups were also proposed based on the phenomena observed.