Abstract The experimental work of Ulrich Franck and his collaborators on phase separation in binary aqueous mixtures of gases at high pressures and temperatures is summarized and placed against the background of the school of Van der Waals, Van Laar and Kamerlingh Onnes in The Netherlands, who performed the first theoretical and experimental studies of such systems around 1900. They discovered and explained critical curves that run to high pressure, as well as gas–gas phase separation and barotropy. The Dutch School demonstrated that a Van der Waals model with weak mutual attraction between the two components readily produces these types of phase behavior. Likewise, Franck and Heilig's modern molecular-based model represents semi-quantitatively, and with few adjustable parameters, the large variety of supercritical aqueous mixtures studied by Franck and coworkers in the second half of the 20th century.