De-esterification is an initial step in the metabolism of certain herbicides, for example, fenoxaprop-ethyl [(+/-)-ethyl 2-[4-[(6-chloro-2-benzoxaolyl)oxy]phenoxy]propanoate] (FE). The ethyl-ester bond cleavage of FE to fenoxaprop acid (FA) by purified enzymes, crude bacterial enzyme preparations, and soils was investigated. In similar experiments fluorescein diacetate (FDA) was used as an alternative substrate. FE stability was pH sensitive in acidic buffered solutions; that is, below pH 4.6, rapid nonenzymatic hydrolysis of the benzoxazolyl-oxy-phenoxy ether linkage occurred, forming 6-chloro-2,3-dihydro-benzoxazol-2-one (CDHB) and ethyl 4-hydroxyphenoxypropanoate or 4-hydroxyphenoxypropanoate. With porcine esterase and cell-free Pseudomonas fluorescens extracts, activity on FE and FDA was most rapid at pH 7.6-8.6 but decreased 80-90% at pH 5.6. Yeast (Candida cylindrica) lipase-mediated de-esterification of FE and FDA was not as sensitive to pH; that is, activity at pH 4.6 was 70% of that at pH 7.6. Short-term incubations (20 h) were conducted in eight soils (pH 4.5-6.9) treated with (14)C-chlorophenyl ring-labeled FE (2 mg kg(-)(1)). In the most acidic soils (pH 4.4-4.5) 25% of the (14)C was recovered as FA, versus 30-40% in moderately acid soils (pH 5.0-5.6) and 55% in neutral soils (pH 6.8-6.9). There was a similar correlation between soil pH and FDA de-esterification. CDHB was formed in all acidic soils with levels 4-fold greater in pH 4.4-4.5 soils than in pH 5. 0-5.6 soils. CDHB was not formed in neutral soils. Results demonstrate some chemical hydrolysis (benzoxazolyl-oxy-phenoxy ether linkage) of FE in acid soils, the sensitivity of enzymatic de-esterification of FE to pH, and the potential of FDA as a colorimetric indicator for esterase hydrolysis of FE.