Abstract Molten salt external transport numbers have been examined with respect to the possible in-fluence of the frit material. Improved measuring techniques revealed a slight temperature dependence for the transference of PbCl2 in Pyrex frits over the range 529 to 610°C, but generally con-firmed the values obtained by most of the earlier workers who used porous glass or related mate-rials. Within experimental error the figures were found to be identical in Pyrex frits of two different pore sizes. On the other hand, significant deviations were detected when a boron nitride frit was used. These experiments confirm that the transport properties of the interfacial layer between the melt and the solid depend on the nature of the solid.