In the aftermath of a fire, investigators often need to determine whether ignitable liquids, such as gasoline or other solvents, were used in starting the fire. This task is made more difficult due to the effects of weathering, primarily through evaporation of volatile compounds and biological degradation. These conditions can alter the chemical signature of the liquids. In the current study, 50 ignitable liquids in the Ignitable Liquids Reference Collection (ILRC) database were weathered by evaporation of 0, 0.25, 0.5, 0.75, 0.9, and 0.95 fractions of the original volume. The same liquids were biologically degraded on potting soil for 0,7,14, and 21 days. Each of the weathered and degraded samples was added as a new record in the ILRC database. The database was modified to link all associated records from weathering and degradation of the same ignitable liquid. The data from all of the samples was reviewed by the ILRC Committee and evaluated regarding the potential impacts of weathering and biological degradation on the interpretation of fire debris data. A "Best Practices" statement prepared by the ILRC Committee will inform forensic practice and policy in laboratories that conduct fire debris analysis. The evaporation rates were also modeled as a function of the fraction weathered and found to follow first-order kinetic profiles.