Changes in sample reactivity and catalyst deactivation during the early stages of the hydrocracking of coal liquefaction extracts, using presulphided supported NiMo/Al(2)O(3)and a dispersed Mo catalyst, have been investigated. The characterisation of the liquefaction extract and the hydrocracking products has been made using size exclusion chromatography (SEC), W-fluorescence spectroscopy (W-F), matrix assisted laser desorption ionisation (MALDI-ms) and thermo-gravimetric analysis (TGA). A complex sequence of events has been resolved involving the simultaneous development of catalyst activity with parallel deactivation due to carbon deposition alongside thermal cracking of the extract. The balance of evidence suggests that the sharp reduction in the rate of reaction early in the hydrocracking process reported in earlier work is primarily caused by loss of sample reactivity.