Members of a new class of amphiphiles with two vicinal hydroxyl groups as a second polar moiety have been synthesized by dihydroxylation of trans-octadecenoates made of natural fatty acids or synthetic fatty acid esters. The phase behavior is studied by measuring surface pressure - area isotherms (pi-A isotherms) and equilibrium spreading pressures (ESP) as well as Brewster angle microscopy (BAM) and fluorescence microscopy. Both, pi-A isotherms and the micrographs of all compounds show a two-phase coexistence region, indicating a first-order phase transition from an expanded to a condensed phase. Depending on the position of the vicinal hydroxyl groups along the hydrophobic alkyl chain, the film pressure in the plateau region as well as the onset of the plateau region differ. Additionally, the position of the hydroxyl groups influences the formation of a stable condensed phase: only the methyl dihydroxyoctadecanoates with no or the largest distance between the two polar groups show an increase in surface pressure at small molecular areas. A molecular model for the processes during film compression is developed, where the two-phase coexistence region is interpreted as the continuous removal of one polar group from the water surface during compression.