Smog chamber/FTIR techniques were used to study the gas-phase reaction of C2F5C(O) with HO2 radicals in 100-700 Torr of air, or O-2, diluent at 296 K. The reaction proceeds by two pathways leading to formation of C2F5C(O)OH and O-3 in ayield of 24 +/- 4% and C2F5C(O)O radicals, OH radicals and O-2 in a yield of 76 +/- 4%. The gas phase reaction of CnF2n+1C(O)O-2 with HO2 radicals offers a potential explanation for at least part of the observed environmental burden of fluorinated carboxylic acids, CnF2n+1C(O)OH. As part of this work an upper limit for the rate constant of reaction of Cl atorns with C2F5C(O)OH at 296 K was determined; k(Cl + C2F5C(O)OH) < 1 x 10(-11) cm(3) molecule(-1) s(-1). (C) 2003 Published by Elsevier B.V.