We investigated whether the length of time fire chemical weathers on a terrestrial substrate before mixing into aquatic environments alters the chemical’s toxicity when encountered by juvenile rainbow trout. We also looked at whether the type of substrate where the chemical is applied affects its toxicity to juvenile rainbow trout after mixing into water. Two fire chemicals (Phos Chek LC95A-R and Phos Chek MVP-Fx) were applied to four substrate media (low organic content soil, high organic content soil, duff, and gravel) and allowed to age on the substrate for 7, 14, 28 or 56 days. At the end of the assigned weathering period, 96-hour assays were conducted by adding water to the dosed substrate and stocking each experimental unit with juvenile rainbow trout. The fish were added to the experimental units after allowing the substrate to settle for 60 minutes. Water chemistry parameters were measured at the initiation of each assay, including water temperature, dissolved oxygen, specific conductance, pH, alkalinity, hardness, and total ammonia. Observations on mortality of rainbow trout were made daily and dead fish were removed upon discovery. The experiment concluded 96 hours after fish entered the experiment.