Steel plates 25 mm thick were fillet-welded to 50 mm thick plates according to good offshore welding practice. The thinner plates were inclined at 90° or 60° to the thicker ones to represent, at full size, the crown or saddle positions of a structural tubular T joint. Slices 4 mm or 10 mm thick were cut from these weldments and the elastic, elastic-plastic and residual plastic strains in the surfaces of these sections were measured using photoelastic coatings and moiré interferometry. The slices were loaded by tensile forces on the 25 mm wide parts, reacted at pin joints near the ends of the 50 mm wide part. The positions and directions of loading were arranged to load the welds in the same way as in a tubular T joint, loaded in tension. Yielding initiated at the weld toes and could be clearly identified in the moiré fringe patterns. It progressed into the plates, being inhibited by the heat-affected zone. Maximum plastic strains also occurred at the weld toes. Measurements of residual plastic strains showed that the actual strain range, which ‘drives’ fatigue failure, differs from predictions based on elastic analyses. Post-weld heat treatment is beneficial, but extending the weld along the plate reduces the strain concentrations much more.