The θ-compositions at 25℃ of polydimethylsiloxane for the binary solvent systems：ethyl benzene-diethyl phthalate and heptane-dioxane were determined by the precipitation threshold method of Elias．They were further confirmed by light scattering measurements to give A2=O and by the intrinsic viscosity-molecular weight relationships to give a=1／2．Weight average molecular weights of eight fractions of polydimethylsiloxane were deter-mined by light scattering in toluene and intrinsic viscosities of these fractions were mea-sured in the above mentioned θ-solvents at 25℃．The data fit the following equations.