Co-pyrolysis of a coal and low-grade organic matter has been reported to produce tars of higher yield and better quality, but few study explored the detailed reaction of volatiles. This work studies co-pyrolysis of pairs of solid organic compounds and a low rank bituminous coal in a thermal gravimetric analysis coupled online with a mass spectrometer (TG-MS) using specially designed TG crucibles, with or without a vertical baffle to separate the solid matter. The different pyrolysis results, mass loss, mass loss rates and gaseous products yields, from the two crucibles are compared and analyzed, and attributed to the difference in volatiles' reaction in the void space between the solid particles. The different volatile release behaviors are correlated with the structure of the solid matter, including the presence of aliphatic and aromatic structure and hetero atoms.