Competition experiments are a useful tool for preliminary study of the linear free energy relationship of organic reactions. This article describes a physical organic experiment for upper-level undergraduates to identify the rate-determining step of the Claisen-Schmidt condensation of benzaldehyde and acetophenone by studying the linear free energy relationship. Substituent electronic effects were studied by competition experiments and the product ratios were determined by gas chromatography. Hammett plots were constructed from the relative rates, and the reaction constants (p) obtained in varying the substituents at the para position of benzaldehyde and acetophenone were 3.09 and 1.59, respectively. The reaction rates were found to be more sensitive to the substituents on benzaldehyde. The positive reaction constants suggested that nucleophilic addition (the first step) is the rate-determining step as the reaction rates were enhanced by increasing the electrophilicity of benzaldehyde as well as the acidity of the [alpha]-H of acetophenone. (Contains 1 scheme, 3 tables and 2 figures.)