Fully premixed burners featuring high air-to-gas ratios are currently used, in order to reduce the NOx emission of domestic natural gas boilers. These burner-boiler combinations are sensitive to variations in operational conditions, which can lead to quite high undesirable acoustic noise emissions. The combustion dynamics leading to this phenomenon are not yet fully understood and optical diagnostic methods can be used for the analysis of these dynamic processes. An experimental study has been performed in a domestic boiler with three low-NOx-burners. Spontaneous C-2 and CH emission of the flame has been recorded as a. function of time and combined with simultaneous acoustic measurements, to characterize the thermoacoustic coupling in the system. The flame motion is determined with an intensified camera systems, velocity fluctuations of the air at the burner inlet have been measured by means of hot-wire probes. The flame position as a function of time is provided by image processing of the C-2 and CH emission images. Characteristic fluctuation frequencies of the flame emission have been extracted by Fourier analysis and compared to the acoustic and velocity signals. The coupling of flame fluctuations with the acoustic emission of the system clearly appears from these comparisons. The pressure signal, recorded by the microphone, is related to the flame surface and center point movements.