Chromium has been substituted successfully in a silicalite structure for the first time in acidic medium. A combined application of cyclic voltammetry (E(1/2) = +588 mV vs. standard calomel electrode) and electron spin resonance reveals that both Cr3+ and Cr5+ are present as part of the structure with octahedral coordination and their reversible transformation within the structure is primarily responsible for the catalytic behaviour.