Photofragments produced following 257 nm irradiation of physisorbed methyl iodide films on MgO(100) have been-studied using resonantly enhanced multiphoton ionization coupled with time-of-flight mass spectrometry. The observed signal is associated with photofragment production from methyl iodide chromophores present in the near surface region. Data on the morphology of the evolving film, obtained by monitoring the variation in photofragment intensity with adsorbate coverage, are consistent with an overlayer that grows by a stochastic process. The dependence of the velocity distribution of methyl photofragments on ejection angle and polarization of the photolysis beam indicates that the film is composed primarily of an antiparallel arrangement of methyl iodide molecules whose C-I bonds are aligned close to the surface normal.