The effect of the addition of iron powder to agglomerated flux on welding performance and the optimum welding conditions for high heat input welding were investigated. The main conclusions obtained are as follows: 1 The addition of iron powder to flux increases the deposition rate and improves welding operability. 2 The arc of the leading electrode was generated at a lower point in the molten metal, between the leading and the trailing electrodes, rather than at its top point. The penetration depth was influenced by the ratio of the area of the groove to that of the molten metal on which the leading arc is generated. 3 The cross sectional profile of the bead was influenced by the current ratio of the trailing electrode to that of the leading. The optimum ratio for avoiding weld defects ranged between 0.70 and 0.80. 4 The flux containing iron powder, together with the selection of optimum welding conditions, allowed tandem-wire one-pass submerged-arc welding to be used for 60 mm thick plates.