In this work, porous carbon materials were prepared from styrene–acrylonitrile (SAN) carbon precursors. These precursors were chemically activated with KOH; activation was carried out at different temperatures to investigate the effect of thermal treatment on the surface and electrochemical properties of the activated materials (A-SAN). Furthermore, nitrogen was doped into the A-SAN materials to evaluate the influence of nitrogen functional groups on electrochemical properties. A-SAN 700 was found to have the highest specific capacitance; it is likely that the extensive microporous structure of this material contributes to its considerable specific capacitance. Nitrogen doping at temperatures above 700°C yields lower-quality materials, which is thought to be due to deformation of micropore structures.