With increasing utilization of various synthetic or natural chemicals, the adverse impact of these chemicals and their degradates is of a concern due to their presence in the environment. However, for most currently used synthetic chemicals, the fate of their degradates are yet to be elucidated. General processes and principles of environmental fate were described. These processes, chemical degradation, biodegradation and photodegradation, affect persistence, and other processes, such as binding, plant/animal uptake, volatilization, runoff, leaching, all influence the movement of a chemical and its degradates in the environment. All processes apply to both parent compounds and their following degradates formed in the environment although degradates might not necessarily behave in the same way as their parent forms.;The fate of two natural monoterpenoid pesticides, thymol and phenethyl propionate, was investigated at a concentration of 10mug/g in water and in soil under laboratorial conditions, Both thymol and phenethyl propionate degraded rapidly with half-lives of 4∼16 days. 2-Phenylethanol and 2-(4-hydroxyphenyl) ethanol were detected as primary degradation products of phenethyl propionate. Over time, a considerable volatilization loss of thymol, but not phenethyl propionate, was found in the one-month study under the experimental conditions. The average radioactivity of bound residues remained below 6%, and mineralization was less than 3% during the whole experimental period.;The aerobic degradation and photolysis of tylosin was conducted in water and in soil. Tylosin A is degraded with a half-life of 200 d in the light in water, and the total loss of tylosin A in the dark is 6% of the initial spiked amount during the experimental period. Tylosin C and D are relatively stable except in ultrapure water in the light. Slight increases of tylosin B after two months and formation of two photoreaction isomers of tylosin A were observed under exposure to light. However, tylosin would probably degrade faster if the experimental containers did not prevent UV transmission. In soil tylosin A has a dissipation half-life of 7 d, and tylosin D is slightly more stable, with a dissipation half-life of 8 d in unsterilized and sterilized soil. Sorption and abiotic degradation are the major factors influencing the loss of tylosin in the environment, and no biotic degradation was observed at the test concentration either in pond water or in an agronomic soil, as determined by comparing dissipation profiles in sterilized and unsterilized conditions.;Two photo-reaction products in water were identified as isotylosin A alcohol (E,Z) and isotylosin A aldol (E,Z). The cross-reactivity of these tylosin-related compounds was tested with a specificity of 26% for tylosin B, 19% for tylosin C, 106% for tylosin D, 121% for isotylosin A alcohol and 46% for isotylosin A aldol compared to 100% for tylosin A. Competitive direct enzyme-linked immunosorbent assay (ELISA) for tylosin detection in water was compared with a high performance liquid chromatography (HPLC) method by measuring the same water samples of tylosin from a water dissipation study. ELISA kits detect the other tylosin-related compounds besides tylosin A, which results in difference of tylosin determination in water.;In order to assess the mobility of tylosin in soil, a soil column leaching study and a simple batch sorption experiment were conducted. Tylosin has strong sorption to various soil with sorption distribution coefficients ranging from 42 to 65 ml/g. Leachability is dependent on soil properties and manure amendment with the range of concentration in leachate detected from 0∼0.27ppb after 4 rainfall events in one month leaching column study. The majority of tylosin was not recovered in the testing system, indicating that tylosin mostly was likely mineralized, or irreversibly bound to solid particles since no major degradation products were detected.