A series of poly(N-acylethylenimines) with hydrocarbon and fluorocarbon side chains varying in length from 6 to 17 carbon atoms were prepared by cationic ring-opening polymerization of 2-oxazolines. The behavior of monolayers of these materials at the air-water interface was studied in a Langmuir trough, and multilayer Langmuir-Blodgett (LB) films were prepared. Both monolayer behavior and LB film formation were strongly dependent on the size and nature of the side chains. Polymers with short hydrocarbon side chains (6-11 carbon atoms) form fluid liquidlike condensed monolayers in which the polymer backbone is at the water surface and the side chains are tilted toward the air. They can be easily transferred onto solid substrates, and uniform LB films can be prepared. Their thickness depends on the length of the polymer side chains. Polymers with longer side chains form only rigid monolayers which tend to crystallize after the initial compression, and LB films of these materials can be prepared only with difficulty. Similar behavior was observed for polymers with fluorocarbon side chains, with the only difference that the same effects are observed at shorter side-chain lengths (usually 3 less carbon atoms).