Pressure‐temperature equilibria curves for a series of some 20 rare‐earth arsenates and rare‐earth vanadates which undergo the reconstructive transition zircon (Zr SiO4) &lrarr2; scheelite (CaWO4) type structure in the region 10 000–60 000 atm were obtained. From these curves, a set of data were extracted representing (with a fair approximation to corresponding states) the change of equilibrium pressure for a transition as a function of the radius of the rare‐earth ion. The result is a V‐shaped curve for both vanadates and arsenates showing a minimum transformation pressure near Dy3+or Ho3+. An interesting even‐odd effect is noted.