In this laboratory experiment, students determine the number-average molar masses and second virial coefficients of polyethylene glycol (PEG) polymers ranging in molar mass from 200 to 1500 g mol–1 using vapor pressure osmometry (VPO). Students assess VPO in relation to accurate molar mass calculations of PEG polymers. Additionally, students use the second virial coefficients to identify any PEG self-interactions and any dependence of the second virial coefficient on PEG molar mass. This laboratory experiment is suitable for a biophysical or physical chemistry laboratory.