Wildfires are natural disturbances, but their frequency and severity are increasing across the western United States due to climate and land-use change. Post-fire runoff can substantially alter stream chemistry by delivering nutrients, sediments, organic matter, and metals from burned landscapes. Although the effects of wildfire on freshwater communities are well documented, the mechanisms linking post-fire water chemistry to biological responses remain relatively unexplored. In particular, there are limited data on the effects of metals from ash on aquatic macroinvertebrates post-fire, though leached ash can release alkaline water with potentially toxic concentrations of a range of metals including aluminum, copper, and selenium. Subsequent dilution and neutralization during mixing with streamwater may alter metal speciation, solubility, biological availability, and toxicity. We conducted a laboratory microcosm experiment to evaluate the immediate post-fire effects of metallic and ionic constituents derived from wildfire ash on the common and ecologically important stream macroinvertebrate, Pteronarcys californica. Solutions were prepared using laboratory-grade reagents to match the dominant metallic and ionic composition of ash leachates. These leachates were generated in the laboratory from ash collected in the Seeley-Swan Valley, Montana. Treatment solution pH was adjusted to match that of the source streamwater used in the experiment. Wild-caught P. californica nymphs were exposed to a gradient of concentrations for three weeks, after which we quantified survival, growth, critical thermal maximum (CTMAX), and metabolic rates. Survival, growth, and metabolic rates did not differ detectably among treatments, despite aluminum concentrations in our treatment solutions as high as 30,000 ppb, and copper concentrations as high as 60 ppb. In contrast, CTMAX showed a modest but clear effect of treatment, with higher leachate concentrations associated with increased CTMAX, which was contrary to our expectation. Overall, the muted physiological responses across treatments suggest that chemical transformations associated with neutralization of ash leachates mediates metal bioavailability and biological effects. This work highlights the importance of chemical context in understanding post-fire stressor impacts and helps inform a more mechanistic understanding of how wildfire-altered water chemistry influences aquatic organisms in streams.