The strain induced phase transformations produced in lead monoxide powders by ball milling at temperatures ranging from -196 to 200~'C have been studied by X-ray diffraction methods. It has been found that either of the two polymorphs, litharge and massicot, was partially converted to the other at all the temperatures investigated, and that, whichever polymorph was the starting material, the end product, after a sufficiently long time, was a mixture of the two polymorphs, both highly strained. The percentage of each phase present was found to be dependent on the temperature of deformation. The results have been interpreted by consideration of the production of the rnicrostrain energy required to overcome the energy barrier to the transformation, and of the shear generated in the mill on the pressuretemperature phase diagram. It is considered that shear stresses can affect the nature of the pressuretemperature phase diagram, in a manner theoretically postulated by Kumazawa. The commercial importance of the results has been briefly outlined.