The optic impedanceZof a medium for the propagation of electromagnetic waves is deduced from the definition of wave impedance. A new optical parameterZocalled “specific optic impedance” is defined. The temperature dependence ofZois examined and an exponential relationship between specific optic impedances at two different temperatures has been derived. The validity of the new relationship is established using the published data on ethyl acetate, cyclohexane, benzene, and a binary mixture of polyvinyl pyrrolidone inN,N-dimethyl formamide solutions from the literature. ©1997 American Institute of Physics.