Ultrafast vibrational dynamics of monolayers from a rhenium carbonyl complex at solid−liquid interfaces is investigated with the help of 2D ATR IR spectroscopy in dependence of the solvation environment. By changing the solvent from methanol to acetonitrile, we observe a structural transition between two states and find that this transition is fully reversible. The two coexisting states are identified by distinct but closely spaced (∼15 cm−1) vibrational bands in the region of the A′(1) symmetric carbonyl stretch vibration. Surface-dilution experiments suggest that the newly formed state in acetonitrile is related to aggregation of molecules at the surface. The vibrational energy transfer rate between the two species (∼30 ps) is used as a structural measure, based on which the morphology is characterized as partially ordered dimers in an otherwise disordered but closely packed monolayer.