We examined the catalytic activity of substituted thioureas in a series of Diels-Alder reactions and 1,3-dipolar cycloadditions. The kinetic data reveal that the observed accelerations in the relative rates are more dependent on the thiourea substituents than on the reactants or solvent. Although the catalytic effectiveness is the strongest in noncoordinating, nonpolar solvents, such as cyclohexane, it is also present in highly coordinating polar solvents, such as water. In 1,3-dipolar cycloadditions, the thiourea catalysts demonstrate only very moderate selectivity for reactions with inverse electron demand. Our experiments emphasize that both hydrophobic and polar interactions can co-exist, making these catalysts active, even in highly coordinating solvents. This class of catalysts increases the reaction rates and endo-selectivities of Diels-Alder reactions, in a similar manner to weak Lewis acids, without concomitant product inhibition.