Phase pure, nanostructured V, Mo and W nitrides and carbides were synthesized and characterized in aqueous KOH and H2SO4 electrolytes. Capacitances for most of the materials exceeded that expected for double layer charging and suggested a pseudocapacitive storage mechanism. With the exception of beta-Mo2C, the materials were stable in KOH and/or H2SO4 electrolytes. Capacitances for VN in KOH and gamma-Mo2N in H2SO4 were the highest and exceeded 200 F g(-1). The charge-storage species were interrogated using an ion isolation method. The results provide unambiguous evidence that OH- was the principal charge storage species responsible for the pseudocapacitance demonstrated for VN in KOH while H+ was the principal species for gamma-Mo2N in H2SO4. (C) 2012 Elsevier B.V. All rights reserved.