Nuclear magnetic resonance (NMR) spectroscopy is commonly used in modern synthetic chemistry to monitor the conversion of reactants to products. Since instruction in the use of NMR spectroscopy typically does not occur until after the introduction of nucleophilic substitution reactions, organic chemistry students are not able to take advantage of this technique to measure and quantify important concepts such as relative electrophile and nucleophile reactivity. A phosphorus-31 ([superscript 31]P) NMR spectroscopy experiment is reported that allows undergraduate students to monitor product formation for a number of different alkyl halide electrophiles and phosphorus nucleophiles. In this experiment, students are able to use reactant-to-product ratios obtained by simple [superscript 31]P NMR spectroscopy to explore the relative reactivities of these compounds. This exercise serves as an introduction to both NMR spectroscopy and the specific advantages of heteronuclear NMR spectroscopy.