This work describes an analytical approach to determine changes in mass distributions and structural properties of biomass/coal cogasification tars, after storage of these samples as a function of temperature, time, and presence of ambient light. The samples examined were tars recovered during cogasification of pine and a sub-bituminous coal (70:30 wt %). It is shown that the approach used was able to reveal even subtle differences in the tar, which occurred during its aging under different storage conditions. Precipitation was observed to occur in the tars after <14 h of storage at >5 degrees C. Size exclusion chromatography and laser desorption mass spectrometry revealed the presence of molecular masses from <200 atomic mass unit Cu) to >2000 u in the tars. UV-fluorescence spectroscopy provided information on extents of conjugation, providing evidence of sizes of conjugated aromatic systems in these tars up to at least 8 rings. The study revealed that the molecules that contained the largest conjugated aromatic systems, rather than the molecules with the greatest masses, were primarily involved in the aging reactions that resulted in precipitation occurring. Different aging mechanisms were identified, depending on whether the tar was stored in darkness or in the presence of ambient light.