(\Lambda) structural investigation of the lead orthophosphovanadate compounds (\mathrm{Pb}{3} \mathrm{P}{2 \mathrm{r}} \mathrm{V}{2 \mid-x} \mathrm{O}{8}) has been performed using neutron and (X)-ray powder diffraction. Results obtained by a Rietveld analysis are presented and compared with previous data on "pure" lead vanadate ((x=0)) and lead phosphate ((x=1)). The influence of the substitution of phosphorus for vanadium on the mechanism of the phase transitions of lead vanadate and lead phosphate is analyzed. For a given concentration (x), a decrease of the distance between the layers of lead atoms is observed when the temperature diminishes. In the vanadium-ind phosphorus-rich compounds a critical value is reached where the lone pairs of the lead atoms repel each other as well as the proximate oxygen atoms: this induces a phase transition from the high lemperature phase. For the other compounds (middle part of the phase diagram) a critical value is not reached and the high temperature phase remains stable down (105 \mathrm{~K}). The possibility of a menoclinic local order inside the high temperature rhombohedral phase is also pointed out. O1093 Acidemic Press, inc.