The h.f. capacitance response of a probe in aqueous solutions of sodium chloride has been studied as a function of the conductance. The results show that there is an optimum cable length, less than the half wavelength, at which the apparent capacitance is least affected by the addition of sodium chloride, and that the effect can be further reduced by suitable cable design. The results are interpreted in terms of transmission-line theory. As a result of this work two possible ways of measuring moisture in building materials are suggested, and some results are given for gypsum plaster and a dense mortar. Maximum errors of 0.05 volume moisture fraction have been achieved for gypsum plaster and for dense mortar when the port liquid at saturation was 0.05 M and 0.1 M respectively with -- AATA