Generating power from HyperCoal is a high- efficiency process in which the organic portion of coal is extracted with industrial solvents at a temperature around 360 degrees C and fed to a gas turbine directly. This study sought to establish a selection index for identifying subbituminous coals that give high extraction yields. Subbituminous coals were extracted at 360 degrees C with flowing industrial solvents, and we investigated the relationship between the extraction yield and the quantity of oxygen-containing functional groups in the coal. The extraction yield with a polar solvent, crude methylnaphthalene oil ( CMNO), increased with the quantity of carboxylate groups bridged by metal cations, such as Ca2+ and Mg2+ (COOM). The correlation coefficient between the extraction yield and the quantity was 0.82. Acid treatment of coal before extraction released COOM cross-links, increasing the extraction yield. These results suggest that the thermal extraction of lowrank coals strongly depends on the cross-links rather than the hydrogen bonds. Therefore, the thermal extraction yields of low- rank coals can be estimated from the quantity of COOM in the original coals. The intercept of the regression line between the quantity of COOM and the extraction yield with CMNO was 57.8%. This value is the average extraction yield for low- rank coals with free COOM.