Activation energy (Ea) distributions and pre-exponential factors (A) of 27 humic coals of varying composition, rank, age and origin are discussed. The petrography and rank of the coals have been correlated to the distribution of Ea and the Hydrogen Index (HI). The HI is positively correlated to vitrinitic and liptinitic constituents; in particular, the vitrinite maceral collotelinite has a strong influence on the HI. Inertinitic constituents have a negative effect on the HI. A higher value of the peak position of the Ea distributions is mainly controlled by the thermal maturity and to a lesser extent by the proportion of collotelinite. A number of other vitrinitic components result in a lower Ea peak position. The peak width of the Ea distributions is broadened by high vitrinite reflectance values and higher proportions of collotelinite and the microlithotypes vitrite and vitrinertite. Liptinitic and inertinitic constituents, in particular the microlithotype clarodurite, narrow the peak width.