Rate coefficients (k) of (CH2OH)-C-center dot, (CH3CHOH)-C-center dot, (CH3)(2)(COH)-C-center dot and Cl-2(-center dot) radical addition to maleic and fumaric acids were investigated between pH 1 and 8. Strong pH dependences observed were attributed to changes in protonation states of acids: H2X, HX and X-2 . k of (CH2OH)-C-center dot, (CH3CHOH)-C-center dot, (CH3)(2)(COH)-C-center dot addition to fumaric acid decreased in the order k(H2F) > k(HF-) > k(F2-) in agreement with the nucleophilic character of reaction. The electrophilic Cl-2(-center dot) radical showed opposite tendency. With maleic acid the monoanion had the highest reactivity towards nucleophilic and the lowest one towards electrophilic radicals. This is attributed to a prevalence of steric over polar effects for HM. (c) 2008 Elsevier B. V. All rights reserved.