We report dielectric [ionic thermocurrent (!TC)] experiments and ionic conductivity of cubic solid solutions of the type Sr1-xYbxF2+x. These combined experiments provide us with new information concerning the ionic conductivity mechanisms which play an important role in solid solutions Sr1-xRxF2+x (R is a trivalent rare-earth ion). From the results it will be concluded that the depolarization peak, which is associated with space charges in a polarized sample, can be connected with bulk ionic conductivity. An interesting feature of the ITC results and the ionic conductivity data is that there are major fluctuations of the dielectric properties and the ionic conductivity as a function of the concentration of Yb3+ impurities. These fluctuations are ascribed to clustering of Yb3+ impurities. Fluctuations of this type have not been observed in earlier investigations, where solid solutions Sr1-xRxF 2+x were treated. It appears that clustering is important for R 3+ ions of the final part of the series of lanthanide ions. For R 3+ ions of the first part of the series of lanthanide ions clustering turns out to be unimportant.