The effects of pulsed nanosecond plasma discharges on H2-air and C2H4-air mixtures are studied numerically using detailed chemistry. The premixed flow is activated by a parallel-plate nanosecond pulsed plasma discharge system with pulse duration ranging between 15 to 130 ns, and pulsing frequency between 5 to 60 kHz. The plasma discharge is modeled using a two-temperature model, with ions and neutral species in thermal equilibrium at the gas temperature, and electrons in thermal nonequilibrium. An energy equation for electrons is solved to obtain electron mean energy self-consistently. Electron transport and reaction rate coefficients are expressed as functions of mean electron energy, and stored in lookup tables. In air, a large portion of input pulse energy is used in electron impact vibrational and electronic excitation of N2, and dissociation of O2. In H2-air mixtures, an additional pathway of H2 dissociation becomes important, and atomic oxygen formation is suppressed. In C2H4-air mixtures, formation of smaller hydrocarbon chains and atomic hydrogen utilizes much of the input pulse energy.