The edge termination of MoS2 and CoMoS catalyst particles is studied by density functional calculations. We show that for structures without vacancies Mo-terminated edges have the lowest edge energies. Creation of vacancies, which are believed to be active sites in these catalyst systems, leads to structures having mainly S-terminated edges. Thus, the results indicate that the shape of the MoS2 and CoMoS structures may be a dynamical function of the reaction conditions. Independent of the type of edge termination, the results indicate that triangular-shaped nanocrystals may be expected in order to maximize the exposure of the favored edges. This is in contrast to the commonly assumed hexagonal morphology, but triangular-shaped MoS2 structures have in fact recently been observed in STM studies of model systems .