Room-temperature film balance measurements were used to study the monolayer stability and molecular conformation of third-through fifth-generation poly(benzyl ether) monodendrons with linear oligo(ethylene glycol) tails at the air-water interface. All the third- and fourth-generation monodendrons studied formed stable Langmuir films, but the fifth-generation monodendrons were not stable at 20 degrees C. Monolayers of the smaller dendritic molecules experienced a repeatable collapse to a multilayer upon sufficient compression. Longer hydrophilic tails increased the stability of the Langmuir films. For third-and fourth-generation monodendrons each additional ethylene oxide repeat unit in the tail increased the collapse pressure by 3.1 and 4.1 mN/m, respectively. The molecular area of the monodendrons at the air-water interface increased linearly with molecular weight. Using a simplified cylindrical model, we determined that the dendritic molecules take a vertically elongated shape at the water surface and become flatter as the generation increases. The diameter-to-height ratio ranged from 1/3 to 2/3 for monodendrons of generation three through five.