Department of Chemistry. M. M. Science College, Morvi-363 642, India Department of Chemistry, Saurashtra University. Rajkot-360 005, India E-mail : phparsania@sify.com Fax: 91-0281-578512 Manuscript received 18 December 2001. revised 19 August 2002. accepted 22 August 2002 R,R',4,4'-Cyclohexylidenediphenyloxyacetic acid (PAAC-1 to PAAC-7, R = H, CH3, Cl, Br. NO2 and R' = H, Cl, Br, NO2) are thermally stable up to 226-320°. Kinetic parameters are interpreted in light of the nature of the substituents. A large and positive magnitudes of ∆S# indicated less ordered transition state than Individual phenoxy acid molecules. The endo- or exothermic DTA transitions supported some physical or chemical changes. Various thermodynamic ·parameters of ethyl esters of the above acids in MeOH-H2O (1 : 1, v/v) have been determined con· dctometrically from second order rate constants at 30, 35 and 40° and are interpreted in terms of the nature of substituents on the reaction rate. The experimental data are in accordance with the bimolecular acyl fission mechanism. A large and negative M' and small A indicate highly ordered transition state as compared to individual ester molecules and It is rate-determining step.