The structure and catalytic behavior of ion-exchanged nickel species supported on Loy Yang brown coal was studied. CO and CO2 evolution were affected by the amount of nickel species loaded. Two peaks of CO evolution appeared, one at 670-770 K, depending on the amount of exchanged nickel species, and the other at ca. 920 K, compared to a broader peak at 870 K for acid-washed coal. The amount of CO evolved at 920 K from a 6.4 wt % nickel-loaded sample was larger compared with that from the acid-washed coal, while the amount of CO evolved from a 0.77 wt % sample was similar. The CO2 evolution peak appearing at 650 K became larger with increasing nickel loading, while the CO2 evolution peak appearing at 820 K for the 6.4 wt % nickel-loaded sample was negligible for the acid-washed coal and the 0.77 wt % nickel-loaded sample. At 0.77 wt % loading, the nickel carboxylate groups have a bidentate structure and aggregate to form metal particles at 770 K. At 6.4 wt % loading, the nickel carboxylate groups have a bridged structure and aggregate to metal pal-tides at 660 K. The catalysis behavior of nickel species is explained by the structure and aggregation behavior of nickel species.