Mixed monolayers of cholesterol and phosphatidylcholines having symmetric, different length acyl chains (10 to 16 carbons each) were prepared at the air/water interface. The partitioning of a fluorescent probe, NBD-cholesterol at 0.5 mol%, among lateral domains was determined by epifluorescence microscopy. The mixed monolayers had cholesterol concentrations of 20, 25, or 33 mol%, and in all these monolayers, lateral domain heterogeneity was observed within a defined surface pressure interval. This surface pressure interval was highly influenced by the phosphatidylcholine acyl chain length, but not by the cholesterol content of the mixed monolayer. The characteristic surface pressure, at which the line boundary between expanded and condensed phases dissolved (phase transformation pressure), and the monolayer entered an apparent phase-miscible state, was about 20 mN/m for di10PC and decreased as a linear function of the phosphatidylcholine acyl chain length to be about 2.5 mN/m for di16PC. During initial compression of the monolayers, the sizes of the condensed phases were generally larger, and to some extent heterogeneous with respect to the size distribution, as compared to the situation in monolayers which had experienced a compression/expansion cycle, which took them above the phase transformation pressure. This suggest that the domains observed during initial compression were not equilibrium structures. This study has demonstrated that both the cholesterol content and the phosphatidylcholine acyl chain length markedly influenced the properties of laterally condensed domains in these mixed monolayers. Since the possibility for the formation of attractive van der Waals forces between cholesterol and acyl chains increase with increasing acyl chain length, and since the phosphocholine head group is similar in all systems examined, the observed differences in domain shapes, properties, and stability most likely resulted from differences in van der Waals forces.