The softening point and Young's modulus of polymethylmethacrylate at low temperature with various contents of dibutyl phthalate have been measured by using a Höpier consistometer and a dilatometer respectively, The relationship between the molar concentration of dibutyl phthalate, N, and the softening point, T, can be expressed by the equation T=112-947N.An explanation for the difference between the coeficient 947 and the value 228-232, previously obtained by Zhurkov, is suggested.