Yields and molecular mass distributions are reported for tars obtained in an atmospheric pressure fluidized bed pyrolysis reactor operated at two different freeboard residence times (0.8 s and 4.5 s). Experimentally, significant tar loss through covalent bond scission for the longer freeboard residence time (4.5 s) was observed to occur at temperatures as low as 500 °C. At higher temperatures (580–750 °C) the molecular mass distributions of the tars appear to shift to lower values. The larger molecular mass fractions of the tars (typically the > 2500 Da fraction) are found in greater abundance in tars thought to have undergone the least extent of secondary reactions. Taken together, the data appear consistent with pyrolysis models describing tar evolution as being primarily limited by bond scission and evaporation of tar precursors.