The second-order rate constants at three different temperatures have been determined for nucleophilic substitution reactions of ethyl bromoacetate with various substituted aliphatic carboxylate ions (XCH₂COO⁻) in 90% acetone-10% water (v/v) mixture and the activation parameters have been evaluated. The rate data have been analysed in terms of electronic and steric effects by using various single and multi-parameterequations. Analysis of the results shows that localized and steric effects are more important than the delocalized effect. The regression coefficient of the inductive component is negative indicating that inductively electron-releasing substituents accelerate the reaction and electron-withdrawing substituents retard it. The steric component corresponds to the reaction being subject to steric acceleration by the substituents. The per cent steric effect has been calculated. The Brönsted type correlation is fair, and theβ_N value has been determined.