The effects of variations in alloying content on the microstructure and mechanical properties of high strength steel weld metals have been studied. Based on neural network modelling, weld metals were produced using shielded metal arc welding with nickel at 7 or 9 wt. %, manganese at 2 or 0.5 wt. % while carbon was varied between 0.03 and 0.11 wt. %. From mechanical testing, it was confirmed that a large gain in impact toughness could be achieved by reducing the manganese content. Carbon additions were found to increase strength with only a minor loss to impact toughness as predicted by the modelling. The highest yield strength (912 MPa) in combination with good impact toughness (over 60 J at –100 oC) was achieved with an alloying content of 7 wt. % nickel, 0.5 wt. % manganese and 0.11 wt. % carbon. Based on thermodynamic calculations and observed segregation behaviour it was concluded that the weld metals solidify as austenite. The microstructure was characterised using optical, transmission electron and high resolution scanning electron microscopy. At interdendritic regions mainly martensite was found. In dendrite core regions of the low carbon weld metals a mixture of upper bainite, lower bainite and a novel constituent—coalesced bainite—formed. Coalesced bainite was characterised by large bainitic ferrite grains with cementite precipitates and is believed to form when the bainite and martensite start temperatures are close to each other. Carbon additions were found to promote a more martensitic microstructure throughout the dendrites. Mechanical properties could be rationalised in terms of microstructural constituents and a constitutional diagram was constructed summarising microstructure as a function of manganese and nickel contents.