We report on the application of ultrafast electron diffraction (UED) to the study of complex molecular reactions, the dissociation of Fe(CO)(5). Snapshots of the diffraction images were recorded at different reaction times. The reaction was initiated by a femtosecond pulse and clocked accurately within 2 ps. The dissociation is complete in less than 10 ps and we identify major product distributions and structures; Fe(CO)(2), Fe(CO) and Fe in the branching ratio of ca. 2:5:5. The C-O distance becomes longer, relative to that of Fe(CO)(5), suggesting a non-systematic weaker C-O bond in Fe(CO)(2) and Fe(CO) fragments. The temporal diffraction-difference method greatly enhances the ability of recording structural changes in UED. (C) 1997 Elsevier Science B.V.