The CS J = 2-1 emission line at 98 GHz has been searched for in 10 diffuse molecular clouds. CS column densities are derived by performing statistical equilibrium calculations for the rotational population distribution which includes collisional excitation by electrons as well as by neutral species. Because CS has a large dipole moment, the electron collisions completely dominate the CS excitation in diffuse clouds, where the electron fraction is high. This result suggests that, in general, molecules with large dipole moments may be relatively more detectable by millimeter emission in diffuse clouds than in dense clouds. A detailed model of the gas-phase sulfur chemistry in diffuse clouds is developed to interpret the observations.