A low rank bituminous coal and four solid organic compounds were co-pyrolyzed in pairs in a thermal gravimetric analysis coupled online with a mass spectrometer (TG-MS) using specially designed TG crucibles which separate the solid matter from each other while allow their volatiles to mix in the free gas space of different volume (or height) above the sample bed. The TG and DTG curves of co-pyrolysis of each pair of the matter are compared with those obtained from pyrolysis of the individual matter in the same crucible and with that calculated from TG and DTG curves of the individual matter by superposition. The gaseous products, including light paraffinic and aromatic compounds in the co-pyrolysis are also studied. It is found that the volatiles' reaction in all the cases studied decreases volatiles' yield, and the extent of decrease gets large with increasing free gas space height and is affected by the molecular structure of the organic matter. The nature of the volatiles' reaction is discussed with respect to adsorption, cracking, condensation and induction.