Fine-particle (6-8 nm) iron-sulfur systems with complex chemical compositions are formed when a 3-nm iron oxide is heated with sulfur in the presence of 9,10-dihydrophenanthrene for 1 h. These systems form small-particle pyrrhotite when heated at 320 degrees C. The fine-particle pyrrhotites are very active hydrocracking catalysts but agglomerate rapidly during reaction at 320 degrees C.