The current view is that metallic materials burn in either the liquid or vapor phase. Evidence for this has been offered experimentally by the visual behavior of the combustion process and theoretically by the criterion proposed by Glassman, which is based on the boiling points and combustion temperatures of metallic materials and their oxides. However, results reported by Sato, et al. with aluminum, and recent tests at the NASA White Sands Test Facility with titanium and aluminum, suggest that metallic materials may undergo transitions and burn in both phases. These results have important implications on how we view the overall combustion mechanism of metallic materials, and are important in the development of burn-resistant alloys. An apparatus that monitors the sample weight change during combustion was used to study the burning phase of metallic materials. Materials that burn in the liquid phase were expected to gain weight while the molten, burning droplets were attached to the samples, whereas metals that burn in the vapor phase were expected to lose weight. Experiments were conducted with titanium and aluminum in oxygen at pressures ranging from 0.07 to 0.55 MPa (10 to 80 psia). The titanium sample burned quiescently and there was a consistent increase in the test sample weight during the growth of the burning droplet, indicating that it burns in the liquid phase. These data are consistent with the Glassman criterion. For aluminum, however, the sample alternated between quiescent and turbulent burning, and the sample weight sometimes increased, sometimes decreased, and sometimes remained constant. This indicates that the phase of combustion was erratic and inconsistent with the Glassman criterion, which indicates that aluminum should burn in the vapor phase. Visual techniques are not sufficient to distinguish between vapor-phase and liquid-phase combustion. The gravimetric technique used in this work provides an important additional means of making this distinction.