Department of Chemistry, Osmania University, Hyderabad-7 A.P. Manuscript received 10 March 1975; revised 26 August 1975; accepted 28 August 1975 The rates of base-catalysed condensation of a series of sixteen substituted benzaldehydes with acetone and ethyl methyl ketone were measured colorimetrically. The reaction follows a bimolecular rate law. The plot of log k/k0 vs σ0 for the meta substituted compounds was linear and the ρ values at different temperatures were evaluated. The effective sigma values (σ) for various para substituents and calculation of individual inductive (I) and resonance effects (R) of different substituents have been attempted with the help of Taft equation.