The effect of high NaCl concentrations in the aqueous subphase on the structure and phase properties of selected n-hydroxystearic acid monolayers is studied. The surface pressure-area (pi-A) isotherms, the BAM (Brewster angle microscopy) images of the condensed phase domains, and the lattice structures obtained by GIXD (grazing incidence X-ray diffraction) of 2-, 9-, 11-, and 12-hydroxystearic acid monolayers spread on I M NaCl aqueous pH 3 subphase are determined. Large differences exist between the monolayer characteristics of stearic acid OH-substituted near the COOH-group (2-position) when both groups act as a monopolar entity and those OH-substituted in the mid-position of the alkyl chain (9-, 11-, 12-position) when the molecule has bipolar character. The pi-A isotherms of the bipolar hydroxystearic acid monolayers have an extended flat plateau region only slightly dependent on temperature. In the case of 2-hydroxystearic acid monolayers, first at T > 30 degreesC an inclined small "plateau" region, which is strongly temperature dependent, indicates the fluid/condensed phase coexistence. Analogous differences between the hydroxystearic acid OH-substituted near the COOH group and those OH-substituted in mid-position of the alkyl chain are also demonstrated by the temperature dependence of the phase transition pressure and the entropy changes for the main phase transition. The comparison of the selected n-hydroxystearic acid monolayers spread, on one hand, on I M NaCl aqueous pH 3 subphase and, on the other hand, on pH 3 water shows that, in general, the characteristics of the T-A isotherms and the structure features obtained on the two different subphases are similar. However, specific differences in the domain morphology and lattice structure are observed and are obviously related to the tendency of high NaCl concentrations to give rise to tighter molecule packing and a reduced polar tilt of the alkyl chain. In the special case of 9-hydroxystearic acid monolayer at 5 degreesC, the phase transition observed at spreading on pH 3 water in the pi-A isotherm at similar to18 mN/m between two condensed phases with a NNN-NN transition disappears at spreading on I M NaCl aqueous pH 3 subphase and only a low-tilted phase toward NN and untilted phase occur.