Brewster angle microscopy, interfacial stress rheometry, and Pi-A isotherm measurements are employed to characterize spread monolayers of two new cellulose derivatives, one containing octadecyl side chains the other p-nitroaniline substituents. Similar monolayer phase behavior is found in both cases. At high surface area, a liquid phase is observed to coexist with a dilute gaseous phase. Monolayer compression eventually leads to complete coalescence to yield a uniform liquid phase. Surprisingly, a homogeneous phase is observed only at surface pressures significantly higher than that characterizing the gas-liquid phase equilibrium. This observation is attributed to dipolar repulsion between neighboring liquid domains. Compression beyond surface areas at which a continuous monolayer is observed provokes a phase transition appearing as either a constant pressure plateau or as changes in slope in the surface pressure area isotherms. BAM images reveal only uniform surface films at molecular areas within the transition region. Surface rheological measurements indicate significant changes in monolayer mechanical properties during compression. Importantly, these changes can be correlated with isotherm shape. In all cases, monolayers exhibit a predominantly viscous behavior at high surface areas where two phases coexist. Homogeneous films achieved upon compression exhibit rheological properties that depend on both temperature and the nature of the side chain substituents. In the case of long alkyl side chains (HPC-C18), the uniform film is found to be predominantly viscous at high temperatures, and more elastic at low temperatures. This observation is interpreted as an indication of the partial crystallization of interdigitated side chains, made possible by bilayer formation. Highly dipolar chromophores incorporated as side chain substituents increase interlayer interactions and interdomain repulsion.