Dipole moments of fourteen monofunctional diamantane derivatives were measured in benzene solution. The values found are almost equal for I-substituted (lla-llg) and 4-substituted (IlIa to Illg) isomers but both are distinctly higher than those of the corresponding adamantane derivatives (I). This trend is not accounted for in terms of the classic Smith-Eyring theory of induced moments which works reasonable well for smaller molecules. Probably some basic assumptions of the theory of dielectrics (a spherical molecular cavity with a dipole in its centre) are no more fulfilled for diamantane derivatives.