Activated carbon fibers (ACF) are prepared from phenol–formaldehyde resin fibers through chemical activation and physical activation methods. The chemical activation process consisted of KOH, whereas the physical activation was performed by activation in CO2. The characteristics of the electrochemical supercapacitors with carbon fibers without activation (CF), carbon fibers activated by CO2 (ACF-CO2), and carbon fibers activated by KOH (ACF-KOH) have been compared. The activated carbon fibers from phenol–formaldehyde resins present a broader potential range in aqueous electrolytes than activated carbon and other carbon fibers. Activation does not produce any important change in the shape of starting fibers. However, activation leads to surface roughness and larger surface areas as well as an adapted pore size distribution. The higher surface areas of fibers treated by KOH exhibited higher specific capacitances (214 and 116 F g−1 in aqueous and organic electrolytes, respectively) and good rate capability. Results of this study suggest that the activated carbon fiber prepared by chemical activation is a suitable electrode material for high performance electrochemical supercapacitors.