Effects of copolymer composition, film thickness, and solvent vapor annealing time on dewetting of spin coated polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films (<20 nm thick) were mainly investigated by atomic force microscopy. Surface chemical analysis of the ultrathin films annealed for different times were performed using X-ray photoelectron spectroscopy and contact angle measurement. With the annealing of acetone vapor, dewetting of the films with different thicknesses occur via the spin-odal dewetting and the nucleation and growth mechanisms, respectively. The PS-b-PMMA films rupture into droplets which first coalesce into large ones to reduce the surface free energy. Then the large droplets rupture into small ones to increase the contact area between PMMA blocks and acetone molecules resulting from ultimate migration of PMMA blocks to droplet surface, which is a novel dewetting process observed in spin-coated films for the first time. (C) 2016 Elsevier Inc. All rights reserved.