Dispersion of the second hyperpolarizability gamma(-omega;omega, omega, - omega) of a series of conjugated diacetylene derivatives measured by single beam Z-scan technique, is presented. The spectrum of gamma is explained by a three-level model involving a one-photon excited state, a two-photon excited state and the ground state. The location of one and two-photon states and corresponding transition dipole moments have been estimated. The influence of various electronic states on the nonlinear optical properties of these derivatives have been discussed. Broad correlations of the nonlinearity with the structure have been drawn.