The DFWM, used as a laser spectroscopy for gas phase trace detection, is receiving a great deal of attention for on- line applications in industrial combustion diagnostics. Low release of NOx requires a complete control of nascent NO formation, i.e. of the combustion chemistry involving the OH radical, and space resolved temperature measurements of each species. Both NO and OH have been detected on small hydrocarbon/air flames by DFWM in forward BOXCARS geometry. NO distribution has been investigated by exciting of (gamma) band, temperature and concentration have been calculated after spectral simulation and calibration on doped flames. HO spectra of the A2(Sigma) + - X2(Pi) , on the fundamental and first vibrationally excited transition, have been used to monitor the radical space distribution and its temperature. Line broadening, shifting and intensity borrowing phenomena related to saturation have been investigated in order to correctly model the spectra. The technique has been used to detect OH band in the combustion chamber of a dry low NOx 130 KW prototype burner, obtaining relative OH concentration profiles. A single shot broad band system, contemporary detection a few OH lines in the transition, has been built to operate in turbulent regime.