Osmotic potential (it.) of aqueous solutions of polyethylene glycol 6000 (PEG-6000) was curvilinearly related to concentration. At given concentrations, AI. increased linearly with temperature. The effects of concentration and temperature on ,1. of PEG-6000 solutions differ from those for most salts and sugars and apparently are related to structural changes in the PEG polymer. Measurements of t'. with thermocouple psychrometers are more negative than those with a vapor pressure osmometer, with the psychrometer probably giving the more nearly correct i1, for bulk solutions. An empirical equation permits calculation of A1, from known concentrations of PEG-6000 over a temperature range of 15 to 35 C. Viscometery and gravimetric analysis are convenient methods by which the concentrations of PEG-6000 solutions may be measured.The use of polyethylene glycol to adjust osmotic potential (s,) requires accurate knowledge of the effect of PEG2 on if. The calculation of i, from freezing point depression is unsatisfactory (9). Thermocouple psychrometry (9,11,13,(19)(20)(21) and vapor pressure osmometry (7,12,17) have been employed; however concentrations used to achieve f, values frequently have not been given (7,(11)(12)(13)(14)17) or were reported graphically (4,9,(19)(20)(21)