The problem of NOx emission from flames is of considerable technological interest. It is known that this emission is influenced by how heterogeneities and in particular by equivalence ratio fluctuations. This topic is studied in the present article for the case of a laminar flame fed by a stream of premixed reactants with a superimposed time-variable equivalence ratio. Experiments and numerical calculations are used to establish relations between the time-varying equivalence ratio, the flame temperature, and NOx emission. In the experiment, the equivalence ratio, is determined from light absorption measurements, the mean and fluctuating temperatures are obtained with thermocouples, and NOx emission is deduced from gas analysis. Time-dependent calculations are carried out with two reaction Schemes and a set of models for the heat losses from the flame and burnt gases. Steady-state measurements and numerical calculations of temperature and NOx are in good agreement for lean flames. When the equivalence ratio takes values close to unity, agreement is obtained by including surface catalycity effects. The time-dependent temperature waveform calculations are in good agreement with measurements. In addition, the time-dependent response of the flame in terms of NOx emission is predicted. A numerical study of the frequency response of the flame shows that the cut-off frequency for the time varying NOx mole fraction is lower than the maximum temperature cut-off frequency, and lower than the spectral content of turbulent flames.