Two-dimensional (2-D) phase transitions of three low-mass-weight liquid crystals (LMWLC) with different mesogens have been studied by;pi-A isotherms at different temperatures, UV-spectroscopy, and Brewster angle microscopy (BAM). The effects of mesogenic unit structures, which are biphenyl or benzoate phenyl ester or ether bonds at various locations in the mesogenic unit, on the bulk phases and the monolayer behaviors have been discussed. The molecules with benzoate phenyl mesogens have existed in the liquid expanded (L-E) monolayer structure phase in which the molecules lie flat on the surface of water because of the strong anchoring effect of the ester group on the water surface, whereas the liquid condensed (L-C) phase is the form in which the molecules are close-packed and oriented perpendicular to the film plane. 2-D phase changes from the L-E to the L-C phase were characterized by UV spectra and observed in situ by using BAM. The experimental results of surface pressure dependence on the area per molecule were in good agreement with the theoretical results which involved the formation of L-C domains dispersed as a superlattice in the continuous L-E phase. The adjustable parameter of used for the best fit of the theoretical results and experimental data is close to both the vertical sections of the molecules in L-E phase and the cross sections of the mesogens in the L-C phase for the molecules. The effect of the functional group in the mesogens on the monolayer behavior is essential for the orientation and spreading behavior of the molecules.