The microscopic motion of anthracene-labeled poly(ethyl acrylate), poly(ethyl methacrylate), poly(octadecyl acrylate), and poly(octadecyl methacrylate) in the air/water interfacial monolayers was examined by the time-resolved fluorescence depolarization method. This method provides direct information on the molecular dynamics based on the rotational micro-Brownian motion of a polymer main chain in a quasi two-dimensional plane. The degree of fluorescence polarization for the anthracene probe was analyzed using the sum of two exponential functions with time constants in a nanosecond range. These polymers formed stable monolayers, while the relaxation time of rotation was dependent on both the main-chain structure and the side-chain length as well as on the applied surface pressure. The steric hindrance of the a-methyl group and the close packing of the long alkyl side chains reduced the mobility of polymer segments in the monolayer and lengthened the rotational relaxation time from a few nanoseconds to a 100 ns. The rotational diffusion coefficients were also evaluated to be on the order of 10(6)-10(7) rad(2)/s.