Empirical formulae are obtained relating the direct band-gap energy of semiconductors and the mean atomic number of the constituent atoms. The formulae are used to indicate doubtful values and to obtain the values of bowing parameters of energy band-gaps of ternaries and quaternaries. Derived values of the bowing parameter give band-gap energies of ternaries and quaternaries agreeing with experiments to within 0.3-7.3% for all III-V compounds except those containing indium antimonide. © 1995.