In this paper, we describe the second-order nonlinear optical properties of a series of 1,3-dithiolebased electron donor-acceptor systems incorporating proaromatic donor and spacer groups. Modification of the proaromaticity of the quinoid spacer gives rise to NLO-phores with mu beta values ranging from -2000 X 10(-48) esu to +3000 x 10(-48) esu. Quite surprisingly, compounds with a p-benzoquinoid spacer and a strong acceptor group show negative mu beta values, usually associated to zwitterionic ground states, and yet they are largely quinoid, as evidenced by crystallographic data and theoretical calculations. Progressive benzoannulation of the spacer and introduction of alkylsulfanyl substituents on the dithiole donor unit result in a shift to more positive mu beta values. DFT and ab initio calculations verify these empirical trends.