Aqueous two-phase systems of poly(ethylene) glycol and a salt are considered as extraction media for large-scale downstream processing of industrially important enzymes. The present paper deals with one of such two-phase systems. New experimental results are presented for the composition of the coexisting liquid phases of the system (water + poly(ethylene glycol) + sodium citrate) at 298.15 K for three poly(ethylene) glycols of different molar masses of about (600, 1500, and 3000) g·mol−1, respectively. The experimental results are described within experimental uncertainty by a semiempirical expression for the excess Gibbs energy.