This work concentrated on very precise density measurements of aqueous twophase systems (ATPS). The relations between density and polymer concentration of solutions of binary systems were determined experimentally at 293.15 K at concentrations ranging from 0 to 20% (w w) of polymers mostly used for ATPS: PEG, dextrans, and Reppal PES. The effect of temperature on density in the temperature range from 29?.15 to 323.15 K was determined for binary dextran 250,000 and PEG 20,000 systems. Equations that describe the relation between density and polymer concentration in polymer water binary systems were calculated and combined for predicting the density of mixtures of ternary systems. When these values were compared with measured densities of ternary polymer systems, no significant difference was found (less than 0.2%, of dextran ). A combination of density and total dry matter analysis was therefore taken as a new method for determining the composition of upper and lower phases after phase equilibria of ternary systems. Perfect agreement from tie line plotting, mass balance, and volume ratio was observed.