The phase separation of dye-labeled poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) (PODMA) in two-dimensional monolayers was investigated by scanning near-field optical microscopy (SNOM). The energy transfer (ET) efficiency between dyes introduced to the polyiners was mapped with SNOM for a PiBMA/PODMA homopolymer blend monolayer. The ET fluorescence images revealed that the phase boundary had a width of a few hundred nanometers. which was considerably larger than that expected in a three-dimensional bulk state. The micro-phase separation of a diblock copolymer PiBMA-block-PODMA was also discussed. Ribbort-like lamellar structures with a width of ca. 300 nm were observed. and the block polymer chain took a highly elongated conformation in two dimensions. (C) 2002 Elsevier Science B.V. All rights reserved.