A new type of concentration dependence,i.e., intersite-distance (ρ) dependence, of the activation energy for the hole drift mobility of the 9-ethylcarbazole-3-carbaldehyde diphenylhydrazone (ECH) and 9-ethylcarbazole-3-carbaldehyde methylphenylhydrazone (ECMH) dispersed in polycarbonate (PC) is reported. The activation energy for charge transport in the ECH or ECMH-doped PC systems gradually decreases with decreasing ρ for large values of ρ, becoming nearly constant for small values of ρ. This behavior contrasts with that for the 4-diphenylaminobenzaldehyde diphenylhydrazone (DPH)-doped PC system. Specific intermolecular interactions between ECH or ECMH molecules having a planar carbazole moiety at relatively high concentrations are suggested to be responsible for this concentration dependence of the activation energy.