SummaryThe fracture toughness (KIC) has been measured (at 1 week) for a selection of sixteen amalgam products. This included low copper (i.e. Cu<6%) and high copper (both single component and admixed) products in milled and spherical powder forms. The results have shown that the fracture toughness decreases progressively as the copper content increases. The admixed high copper products possess superior fracture toughness to those containing a single component powder. These results have been attributed to differences in the microstructures produced by changes in composition and form in which the copper addition is made.