We have probed the conformational stability of cellular retinoic acid-binding protein I, a predominantly beta-sheet protein, using hydrogen/deuterium (H/D) exchange in solution. Transiently populated intermediate states were detected using H/D exchange measurement under mildly denaturing conditions (pH 2.5 and room temperature). By inducing collisionally activated dissociation in the nozzle-skimmer region of the electrospray source of an FT ICR mass spectrometer (MS), residue-specific information was obtained as to the degree of protection of backbone amide hydrogen atoms as a function of exchange time. The measurements do not appear to be influenced by intramolecular proton mobility in the gas phase. Multiply charged fragment ions covering half of the protein sequence were readily assigned using the extremely high resolution of FT ICR, allowing in some cases protection at individual amide hydrogen atoms to be measured. The results reveal distinct structural regions featuring very different backbone protection patterns. The high data acquisition rate of the FT ICR MS results in significant improvement of temporal resolution over NMR spectroscopy.