Ionic conductivity of a series of fluorite-type solid solutions Pb1-xYxF2+x (0.007 less than or equal to x less than or equal to 0.222) has been measured in the frequency range 100 Hz - 3 MHz and in the temperature range 25-250 degreesC under vacuum. The conductivity decreases and the activation energy increases gradually upon increasing the yttrium content up to similar to4.5 mol % of Y3+ whereas, beyond that and up to 22 mol % of Y3+, the ionic conductivity increases and the activation energy decreases. Thus two distinct regions in the general composition Pb1-xYxF2+x could be delineated, in one the polarizability factor predominates over the number of extra-interstitial F anions and vice-versa in the other region. These results indicate the competing effects between the disorder caused by interstitial F anions and the polarizability of the host and guest cations,