Mass spectrometric measurements often begin with the determination of the masses of ions, assuming that these are set and stable. But we can also monitor the progress of mass shifts purposefully created in our experiments, and derive reaction kinetic information. For example, labile hydrogen (H) atoms in molecular and ionic structures can be replaced with deuterium (D), and the mass shifts and kinetics can be followed as the measured ion masses progressively increase. In its current state of development, H D exchange mass spectrometry provides insights into complex protein structures and their dynamics through the study of the rates of exchange under various conditions.