Thixotropy is a reversible sol-gel transformation in which a gel can be liquefied, and thus changed to the sol condition, by mechanical disturbance—such as shaking or stirring—and after agitation ceases will, with time, again revert to the gel condition. All thioxtropic materials so far investigated have been two-phase systems, of which one phase is generally solid and the other liquid. The present investigation deals with measurements of the thixotropy of paints undertaken to arrive at a more complete understanding of their flow characteristics, especially under the influence of extremely small shearing forces. While paint has long been recognized as a plastic material, its thixotropic nature has not been generally appreciated, although “false body” observed in some paints is a manifestation of thixotropy. Williamson attempts to calculate a “false body constant” for any paint, basing his calculations upon plasticity data, and failing to realize that the presence of “false body” in all paints requires all paints to be thixotropic. In addition Williamson has failed to appreciate the full significance of the mutual presence of plasticity and thixotropy. As has been previously pointed out by the author, Williamson's treatment of the subject is far from satisfactory in the practical case of evaluating leveling ability of paints. Other investigators of the plasticity of paints have, in general, either not recognized the presence of thixotropy or else ignored it.