Angle-resolved two-photon photoemission was used to study the energy relaxation, population decay, and localization dynamics of image potential state (IPS) electrons in ultrathin films of dimethylsulfoxide (DMSO) on an Ag(111) substrate. Dynamic energy shifts of 50 ±10 meV and 220 ±10 meV were observed for n = 1 IPS electrons at one monolayer and two monolayer coverages of DMSO, respectively. The difference in energy shifts is attributed to rotational hindrance of the molecular dipole in the chemisorptive first monolayer. The finding confirms the proposed mechanism for the low differential capacitance of dimethylsulfoxide at noble metal interfaces in solution. A novel description of the IPS as a surface capacitance is presented to facilitate comparisons with electrochemical systems.