Dilution represents the contribution from base material to weld metal and its important effects are complicated for multi-pass steel welds. In this study, the dilution was determined for each bead in a 3-pass gas tungsten arc (GTA) weld and a corresponding submerged arc (SA) weld. Different filler materials were used with each process, but both cases involved the deposition of filler wires into grooves in low-alloy (SA508) ferritic steel plates. Martensite and bainite were observed in the heat-affected zone (HAZ) of the base material in both cases. Acicular ferrite dominated the SA weld metal, while this phase was mixed with martensite and bainite in the GTA weld metal. Thermal-metallurgical modelling was performed and it correctly predicted the microstructures in the base-material HAZ for both the GTA and SA weldments. It also captured the GTA weld metal microstructure, when transformations from austenite to acicular ferrite and bainite were treated as being equivalent. However, this approximation led to overestimation of the martensite volume fraction in the SA weld metal. The modelling revealed that increases in dilution favour martensite formation but suppress bainite/ferrite transformations, owing to the melting of greater quantities of the base material, which has a relatively high hardenability.