The effect of temporal variations in equivalence ratio on the NO_x emissions of a premixed methane-air flame was measured in a burner. The NO_x emissions are compared among steady flames with spatially uniform equivalence ratio distributions, steady flames with spatially nonuniform equivalence ratio distributions, and unsteady flames with temporal equivalence ratio fluctuations. Time-varying equivalence ratio was measured optically, time-varying temperatures were measured with thermocouples, and mean NO_x emissions were measured by probe sampling and a chemiluminescent analyzer. These measurements quantify the effect of temporal unsteadiness and spatial nonuniformity of equivalence ratio on NO_x emissions. For lean flames, both spatial nonuniformities and temporal fluctuations in equivalence ratio contribute to an increase in NO_x emissions with respect to steady uniform flames at the same mean flame temperatures. For lean flames, higher amplitude temperature fluctuations result in larger increases in NO_x with respect to steady flames. The dissertation also describes the optical technique for nonintrusive temporal measurements of equivalence ratio fluctuations and techniques for thermocouple compensation at frequencies up to 10 Hz. / Ph. D.