As part of its water-jet-assisted rock cutting research, the Bureau of Mines has initiated a study into the mechanism of rock fragmentation by water-jet-assisted mechanical tools. The objective of this research is to increase coal extraction efficiency by seeking an improved understanding of the synergistic relationship between mechanical cutters and water jets, enabling the design of more effective cutterheads. Rock chips collected during laboratory traverse speed jet-assisted cutting tests were analyzed with respect to the size distribution of the chips and the forces measured during cutting. As the water-jet pressure was increased, the size distribution of the product became coarser with v-face-type cutting tools producing less fine material than conical cutting tools. Fracture mechanics theory accounted for less than 10 pct of the energy consumed during fragmentation of the rock. The energy consumed by processes other than rock breakage decreased with the use of water jets and increasing advance rate.