Secco and Usha [Solid State Ionics 68 (1994) 213] have recently discussed conductivity data for some sulphate-based compositions in the context of a "percolation-type of ion transport mechanism". However, when doing so, they make no distinction between stoichiometric compounds, solid solutions and two-phase regions. Furthermore, they speak of correlations between conductivity and structure "free" volume without presenting any facts concerning the latter concept. E.g. no comparisons are made of the lattice constants or densities of the discussed samples. Their presentation of the underlying experimental data is thus questionable, and their approach offers no advantages when compared with previous concepts. Thus, the conductivity enhancements characteristic for aliovalent solid solutions are due to generally accepted vacancy mechanisms, while the observations for phase mixtures can be associated with the concept of composite electrolytes.