A key component of cryopreservation protocols is the use of cryoprotectants to mitigate damage caused by ice formation. Hydroxyethyl starch (HES) has been shown to be an effective cryoprotectant for erythrocytes; however, little is known about its thermodynamic solution properties. The osmotic virial equation has been shown to be versatile in its ability to describe the solution properties of many types of solutes in aqueous systems including sugars, electrolytes, cryoprotectants, macromolecules, proteins, alcohols, starches, etc. Recently, the osmotic virial equation has been used to fit HES and NaCl aqueous solution data. This study investigates fitting vapour pressure osmometry measurements of three different modifications of HES with the osmotic virial equation. HES modifications were measured after dialysis (membrane cut off: 10,000 g/mol) and freeze-drying at different mass ratios of HES (up to 50%) and NaCl (up to 25%) with 3 different HES modifications (weight average molecular weights (g/mol)/degrees of substitution: 40,000/0.5; 200,000/0.5; 450,000/0.7). Nine models were proposed based on the appearance of the osmotic virial coefficients. The data was then fit to these models for virial coefficient dependence on mass ratio of HES to NaCl and one of the nine models was chosen as the best model based on the lowest sum of squared errors and standard error. The best model for all three HES modifications was of the form B HES = a + b R c and C HES = d , where B HES and C HES are, respectively, second and third osmotic virial coefficients related to HES-to-NaCl mass ratio R, by HES-modification-specific parameters a, b, c, and d. The osmotic virial equation accurately represents osmolality data for aqueous HES-NaCl solutions for the three modifications for a broad range of mass ratios of HES to NaCl, using only a small number of fitting parameters. This research was funded by a Dean’s Research Award for Jingjiang Cheng from the Faculty of Engineering at the University of Alberta, and by the Canadian Institutes of Health Research (MOP 86492). J.A.W.E. holds a Canada Research Chair in Thermodynamics.