The wetting behavior of thin films of symmetric poly(styrene-block-methyl methacrylate) was investigated on self-assembled monolayers (SAMs) of octadecyltrichlorosilane (OTS) that were chemically modified by exposure to X-rays in the presence of air. The concentration of aldehyde and hydroxyl groups on the surface of exposed OTS increases with increasing dose. The polarity and surface tension of the SAMs were tuned to control the wetting behavior of the polymer films. Symmetric, neutral, and asymmetric wetting of the films were observed for doses of 400−1000, 1200, and 1400−2000 mJ/cm2, respectively. The wetting behavior was qualitatively described by estimating the interfacial energies between each block of the copolymer and exposed SAMs and the surface tensions of both blocks using the Fowkes−van Oss−Chaudhury−Good model of surface tension. These results lay the foundation for controlling the morphology of thin films of block copolymers over macroscopic dimensions by nanopatterning substrates with regio...