As glaciers in the western United States thaw in response to warming, they release dissolved organic matter (DOM) to alpine lakes and streams. Biological availability of DOM from small mountain glaciers is unknown. Differences in DOM bioavailability between glacier types like rock and ice glaciers remains undefined, yet rock glaciers outnumber ice glacier approximately ten to one at low latitudes. To assess which components of aquatic DOM are most reactive and the potential for glacial DOM from low latitude glaciers to subsidize heterotrophy in alpine headwaters we evaluated reactivity and molecular composition of DOM from ice glaciers and rock glaciers from four paired catchments (each with a glacier and rock glacier at their headwaters). Biological reactivity was linked to molecular composition by evaluating the chemical characteristics of each DOM pool pre- and post-incubation using common microbial community laboratory assays paired with untargeted mass spectrometry-based metabolomics. Glacier and rock glacier DOM was similar in concentration and chemodiversity, but differed in composition. When incubated with a common microbial community, DOM from ice glacier meltwaters contained a higher proportion of bioavailable DOM (BDOM), and resulted in greater bacterial growth efficiency. Differences in DOM reactivity between glacier types was determined by differences in the relative abundance of only a few dozen compounds. Though BDOM was lower in rock glaciers, because rock glaciers are more abundant and are expected to have greater longevity, we propose that both glacial types will be important sources of bioavailable DOM to alpine headwaters over the coming years to decades.