This study aims to provide a better understanding of the factors that control weld metal strength and toughness for a series of mechanised X100 girth welds produced using a range of narrow-gap gas-metal arc welding procedures. The attainment of high yield strength (> 810 MPa) is considered to be an important target to ensure overmatching strength relative to the longitudinal property distribution of X100 line pipe steel. Key components of this work relate to the reliable measurement of weld metal strength variation through-thickness and as a function of position around the circumference of a given girth weld. The results obtained for the two X100 narrow gap mechanised gas metal arc welds illustrate how dramatically different the weld metal tensile properties and Charpy impact toughness can be for what might be considered small changes in welding conditions. Although the welds were produced using the same electrode (C-Mn-Si-Ni-Mo-Ti) wire, differences in energy input, interpass temperature, and weld width (offset distance) were considered responsible for the major differences in yield strength and notch toughness. The very high yield strengths, together with Charpy impact toughness levels in excess of 200 J measured for the Narrow Offset weld suggest that the targeted yield strength of 810 MPa combined with good low temperature toughness should be readily achievable provided appropriate control of the welding procedure is observed. Results from the series of single and dual torch welds provide further evidence of this state merit with yield strengths consistently higher for round bar compared to corresponding strip tensile specimens. For this series of welds, some interesting trends were observed. Clearly the strength of the single torch welds was considerably higher than those obtained for the dual torch welds. In all cases very good weld metal toughness was observed. It is worth pointing out that the tensile properties of the single torch welds would generally satisfy an overmatching strain based criterion for X100, whereas the dual torch welds would be suitable for both a stress based criterion for X100 or overmatching criterion for X80 high strain design.