In order to explore the degree of contact between hydrophilic blocks and the substrate, the dewetting behavior of Langmuir-Blodgett (LB) films of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) induced by PMMA-selective acetone vapor were investigated by atomic force microscopy (AFM) for the first time. With the annealing of acetone vapor, the LB films of PS-b-PMMA undergo the swelling and coalescing of aggregates, the formation of bicontinuous patterns, the formation of droplets, and the periodic increase and decrease of droplets. The emergence of the bicontinuous patterns indicates that the dewetting occurs via the spinodal dewetting mechanism. The periodic droplet evolution is a novel phenomenon observed for the first time and quite different from the single droplet evolution of spin-coated films, which is probably due to the degree of contact between PMMA blocks and the substrate in the LB films being larger than that in the latter. (C) 2015 Wiley Periodicals, Inc.