Metastable Atom De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with unparalleled surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly, for the first time, by use of spin-labeling techniques. The electron spins on the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Results are reported for Cu(100) and Ni(111) surfaces under a variety of surface conditions. The data indicate that metastable atom-surface interactions may be more complex than is generally assumed and thus suggest that assumptions inherent in earlier analyses of MDS spectra merit additional investigation. The present work further demonstrates the power of spin-resolved measurements in the study of particle-surface interactions and in surface spectroscopies in general.