The rationale and approach that underlie the use of mathematical models to forecast the environmental behavior of organic chemicals are examined. The general concept are then used to show how knowledge of environmental and dye chemistry can be used to suggest pathways that are likely or unlikely to be determinants of dye fate in aquatic systems. Reactions involving precipitation of Ca and Mg salts and ion exchange with sediments may be important for anionic and cationic dyes, respectively. The uncharged dyes are considered in regard to possible volatilization, sorption, and bioconcentration. Suitable kinetic and equilibrium constants are not currently available for quantitative prediction using mathematical models.