Conductivity measurements have been performed on the solid solution series, Na3Zr2 - x/4Si2 - xP1 + xO12, for 0 < x < 2. These materials are Zr-deficient nasicon. A monoclinic to rhombohedral phase transition occurs around x = 0.333. The variation of the ionic conductivity as a function of the composition x is investigated in the temperature range 150-600 K. It is shown that the bulk ionic conductivity decreases slightly from 6 x 10(-4) S cm(-1) for the Zr non-deficient nasicon (x = 0) to 6 x 10(-5) S cm(-1) for the Zr highest-deficient material (x = 1.667). The presence of the above mentioned structural change does not affect the ionic conductivity. The mobility of the Na+ ions in these materials seems to be mostly influenced by the size of the bottlenecks through which the ions have to pass. The variations of both the ionic conductivity and the activation energy of the ionic motion process in the bulk of the material can be explained by structural considerations.