Surface pressure-area per molecule (Pi-A) isotherms and lattice structures of n-hydroxyoctadecanoic acid monolayers with n = 2, 9, 11, and 12 are experimentally studied to obtain information on the effect of the hydroxyl group position on the phase behavior and properties of the monolayers. Striking differences exist between the monolayers of 2-hydroxyoctadecanoic acid and hydroxyoctadecanoic acids with n = 9-12. Theoretical calculations of the Pi-A isotherms are performed to explain the strong effect of the position of the OH-substitution on the thermodynamic properties of the monolayers. The calculations provide reasonable agreement between the theoretical predictions and the experimental n-A isotherms are obtained as well as between the parameters calculated from the theoretical model and those obtained from the GIXD measurements. The combination with the experimental GIXD data allows conclusions on the orientation of the molecules in the different monolayer states and their changes at the main phase transition. For 2-hydroxyoctadecanoic acid monolayers, a disordered packing is found in all monolayer states, whereas for (9-12)-hydroxyoctadecanoic acid monolayers, well-ordered 2D lattice structures stabilized by hydrogen bonding between the OH-groups are formed. The data analysis indicates that at OH-substitution in the 2-position the neighboring polar group acts as a monopolar entity. At OH-substitution in the (9-12)-position both polar groups lead to bipolar behavior. At the main phase transition, the weaker polar OH-group is forced out of the water interface and well-ordered 2D lattice structures stabilized by hydrogen bonding between the OH-groups are formed. 9-Hydroxyoctadecanoic acid monolayers show an interesting second phase transition at Pi = 18 mN/m between two condensed phases which can be satisfactorily described by the theoretical model which assumes twodimensional compressibility of the condensed monolayer.