The critical coordinates of seven poly(ethylene glycol)/water systems with different molar masses of the polymer have been determined by demixing experiments. From the critical point data, values of the interaction parameters of the Flory-Huggins-Staverman-van Santen theory and their extensions have been calculated and the Theta-temperature of the system has been extrapolated. The consistency of the results is discussed with regard to the known findings of earlier investigations. The averages of the molar masses of the polydisperse polymers have been deduced from data of membrane osmosis, light scattering, and time-of-flight mass spectroscopy. Although all polymer samples are polydisperse, it has been found that the scaling law phi(p,c)(r) = K(1)r(x) (-j) with j approximate to 0.38 is obeyed within the range of the experimental errors. The scaling amplitude KI is nearly unity. By using the determined Theta-temperature, another predicted scaling law, Theta - T-c(r) = K(2)r(x)(-k), has been tested. Although it is very sensitive to all scattering of the experimental data the exponent k is found in the range of reasonable values.