Lateral compressibility and miscibility phase behavior of monolayers of phospholipids and cholesterol were investigated using a Langmuir film balance and epifluorescence microscopy. Three lipid mixtures were used: dihydrocholesterol and either DMPC [di(14:0)PC], DLPC [di(12:0)PC], or 1/1 DMPC/DPPC [1/1 di(14:0)PC/di(16:0)PC]. The mixtures were chosen to produce liquid phases and a variety of miscibility phase diagrams. No general correlation was found between changes in lateral compressibility and miscibility phase transitions, even at miscibility critical points. A previously reported correlation for a monolayer system of cholesterol and DMPC seems to be coincidental. These results strongly argue against using only pressure-area isotherms to determine miscibility phase transitions in liquid monolayers.