The band gap, EG, of an organic solid can be estimated by adding the magnitudes of oxidation and reduction potentials of the compound in solution, plus a correction to the ion solvation energy which allows for the different dielectric constants of solvent and solid. Values of EG for 30 organic solids have been estimated. ��� The auto-ionization model, previously used for aromatic hydrocarbons, is applied to dyes and chlorophylls. The values of EG are used to interpret various existing results on photoconduction and semiconduction in dyes and chlorophylls.