By combining the surface sensitivity and ultrafast temporal resolution of ultrafast electron crystallography (UEC) with in situ surface preparation, adsorbate deposition and surface characterization, we extend the capability of UEC to monitor structural dynamics of monolayer adsorbates. Here, we report investigation of the Ni(100)-c(2 x 2)-S, Ni(100)-p(2 x 2)-O, and Ni(100)-c(2 x 2)-CO systems. Their structural dynamics are quantified by comparing the temporal change of the experimental electron diffraction patterns to the theoretically-obtained multi-scattering simulations. This Letter paves the way to future exploration of monolayer reactivity under controlled conditions of ultrahigh vacuum and with spatiotemporal resolutions at the atomic scale. (C) 2012 Elsevier B. V. All rights reserved.