The structure and dipole moments of both 3- and 4-phenylsydnones along with derivatives containing both the dimethylamino and nitro groups have been calculated using the ab initio 3-21G basis set. The presence of the electron donor in the phenyl rings results in an increase in the magnitude of the dipole moment while the electron attractor has the opposite effect. The nonlinear optical properties of these structures have been assessed by calculating their molecular hyperpolarizabilities using a sum-over-states semiempirical approach. The results obtained show that the hyperpolarizabilities of the two isomers differ greatly with the dimethylamino group enhancing the value for the 3-phenyl derivative and the nitro group enhancing the value for the 4-phenyl derivative. The largest values are calculated for 4-phenylsydnones.