Water activities of binary and ternary poly(ethylene glycol)s (PEGs) solutions, at 298 K, were measured with an Aqua Lab CX-2 dew point device. The concentration was varied in the range 5-90 wt% or the solubility limit for each polymer (PEG molecular weights 200, 400, 600, 1000, 1450, 6000, 8000, 10 000 and 20 000). The group contribution method UNIFAC was used for correlating and predicting the experimental data of PEG solutions. Using own experimental data of binary solutions and some data obtained from literature sources, the interaction parameters for a new specific group [-CH2CH2O-] were estimated including a linear temperature dependence. This correlation provides a 0.527% mean deviation between experimental and calculated a(w) values. The prediction of water activity for ternary systems results in mean deviation of 0.290%.