Alloy 718 suffers from microfissuring in the weld heat-affected zone, and compositional, structural and mechanistic causes of this defect have been examined. Both bulk sulfur and bulk carbon increase microfissuring, and heat treatments typical ofhomogenization, solution annealing and age hardening reveal that microfissuring is sensitive to the microstructural and chemical distributions established during heat treatment. Increased grain size increases microfissuring more than heat treatment or bulk chemistry. The mechanistic cause of microfissuring, constitutional liquation of niobium carbide and Laves phases, produces intergranular liquid films with wetting angles that are dependent on the chemical composition of the grain boundary region prior to welding. Microfissuring also correlates with the temperature dependence of the intergranular liquid wetting angle in the heat-affected zone.