Ninety‐six hour static bioassays were conducted with one‐to‐six‐day‐old fathead minnow fry to determine the acute toxicity of leachates generated from Lurgi gasification ashes, an H‐Coal liquefaction residue and an SRC‐I liquefaction residue. LC‐50 values, their 95% confidence intervals and the amount of dilution necessary to eliminate mortality during the 96‐hour bioassays were determined. Leachates that were generated from the solid wastes at four pH levels and under anaerobic and aerobic conditions were analyzed for approximately 40 chemical constitutents. Nineteen of the constitutents were found to exceed recommended water quality criteria levels in some of the samples at pH values between 2.3 and 11.3. All eight leachates, including the unadjusted pH leachate, generated from the Lurgi ash of an Illinois No. 6 coal contained concentrations of Ca, F, Li, Mn, Pb, SO4and Sb which exceeded recommended water quality levels. Leachates produced from the gasification ashes were not acutely toxic at the unadjusted pH levels; however, unadjusted pH leachates from the liquefaction residues were acutely toxic. All leachates with a pH less than 6.2 were acutely toxic and required at least a 1:10 dilution and as much as a 1:2000 dilution to eliminate mortality. Mortality was probably caused by the combined effects of pH and total ionic strength of the leachates.