Investigations were carried out to evaluate the influence of dissolved nitrogen inferritic steel weld metal on its toughness and strain aging behaviour through fracture mechanics as well as conventional impact testing approaches. Manual metal arc C-Mn steel weld metals with nitrogen contents between 80 and 210 ppm (wt) were investigated under four different post-weld conditions, namely, (i) as welded, (ii) stress relieved, (iii) artificially strain aged, and (iv) artificially strain aged and stress relieved. Quantitative metallography and low load microhardness studies of microphases were an integral part of these investigations. The results demonstrate the highly detrimental effect of nitrogen on the toughness behaviour of C-Mn steel weld metal, particularly under strain aging conditions. This is substantiated by the decrease of acicular ferrite content with the accompanying increase in primary ferrite and ferrite with second phases in the microstructures. Also, there is a distinctive increase in acicular ferrite microhardness. Post-weld stress relieving heat treatment under these conditions results in only a marginal improvement in toughness and shifts the fracture behaviour from brittle to ductile or quasiductile only for low nitrogen weld metals. Comparing the results of the crack tip opening displacement and Charpy tests, it is observed that both methods show the influence of nitrogen on toughness behaviour to have the same form but the magnitudes of the effect measured are different, the results obtained using the fracture mechanics method appearing very conservative. © 1998 The Institute of Materials.