Coprocessing reactions were carried out using five coals and three petroleum residua at 350, 400, and 450 degrees C with N-2 and H-2. No catalyst was used. The interactions between the coal and the petroleum residua were investigated in terms of following the conversions and yields of THF-insolubles. The influence of temperature, time, petroleum residua, and coal characteristics was examined. Conversions at or below 400 degrees C are primarily determined by the nature of the coal. The characteristics of the residuum have little influence on the coal conversions. The residuum behaves like a liquid medium for the coal fragments at this temperature regime and does not have properties of a hydrogen donor or hydrogen shuttler. The maximum coal conversion is approximate to 40 wt %, comparable to the conversions obtained in pyrene under the given conditions. At temperatures >400 degrees C retrogressive reactions are observed, indicated by low or even overall negative conversions (-20 wt %). The retrogressive reactions are a result of the interactions between the coal and petroleum residuum fragments and are influenced by the properties of the coal as well as the petroleum residuum. Both the coal and the residuum must be undergoing thermal decomposition for the retrogressive reactions to occur. Residua that have a higher degree of polycondensation (H-ar/C-ar less than or equal to 0.4) may be inferior coprocessing solvents at temperatures >400 degrees C. Similarly, noncaking coals proved to be less desirable than caking coals.