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Revised Manuscript Received April 15, 19948 abstract: The hydrophobicity profiles across phosphatidylcholine (PC)-cholesterol bilayer membranes were estimated in both frozen liposome suspensions and fluid-phase membranes as a function of alkyl chain length, unsaturation, and cholesterol mole fraction. A series of stearic acid spin labels, with the probe attached to various positions along the alkyl chain, cholesterol-type spin labels (cholestane and androstane spin labels), and Tempo-PC were used to examinedepth-dependent changes in local hydrophobicity, which is determined by the extent of water penetration into the membrane. Local hydrophobicity was monitored primarily by observing the z component of the hyperfine interaction tensor (Az) of the nitroxide spin probe in a frozen suspension of the membrane at-150 C and was further confirmed in the fluid phase by observing the rate of collision of Fe …