The effect of 3% O-2 addition to the fuel on detailed chemical structure of a 1,3 butadiene counter-flow diffusion flame has been investigated by using heated microprobe sampling and online gas chromatography mass spectrometry. Centerline gas temperature and species mole-fraction profiles were measured both in the absence and presence of oxygen on the fuel side. The rapid thermocouple insertion method was used to obtain the flame temperature profiles. Although the addition of oxygen to the fuel side did net significantly change the shape and peak values of the temperature profiler, species mole-fraction profiles were significantly altered. Flame reactions started earlier in the presence of oxygen in the fuel, resulting in the shift of the reaction zone toward the fuel burner port. The presence of oxygen in the fuel Led to decreased peak mole fractions of the aromatic and two-ring polycyclic aromatic hydrocarbon species. In contrast, the peak male fractions of polycyclic aromatic hydrocarbon having three or more rings significantly increased in the presence of oxygen in the fuel stream. An analysis of the data suggests the need to invoke nov reaction mechanisms describing the formation and destruction of polycyclic aromatic hydrocarbon.