Nucleophilic addition to 1,3-dimethyl-5-pheny1-1,4,5,6-tetrahydropyrimidinium ion provides a quantitative measure of stereoelectronic control. This amidinium ion presents the nucleophile with two distinct paths for attack. Axial attack is favored by interaction between the orbital of the developing bond and antiperiplanar lone pairs on the nitrogens. Reaction of the amidinium salt with diverse nucleophiles (D-, H3C-, n-Bu-, PhCH2-, allyl(-), Ph-, C5F6-. CH2=CH-, HC equivalent toC(-), PhC equivalent toC(-), CN-) produces mixtures of cis and trans stereoisomers. Both kinetic and thermodynamic product distributions were measured by H-1 NMR, before and after acid-catalyzed equilibration. The values provide insight into the roles of steric and stereoelectronic forces at the transition state and in products. Stereoelectronic effects on reactivity are found to be weak (ca. 1 kcal/ mol).