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The nature and the effect of the unmixed zone in dissimilar fusion welds, especially its significance with regard to corrosion resistance, has been studied in this Welding Research Council-sponsored program. Three types of super-austenitic stainless steel (high Mo) (AL-6XN, 254SMO and 654SMO) and five types of fully austenitic nickel-based weld filler metals (Inconel 625, C-22, C-276, 686CPT, and P16) were evaluated. Shielded metal arc (SMA), gas metal arc (GMA), gas tungsten arc (GTA) (filler added), and gas tungsten arc (GTA) autogenous welding processes were used for fabricating weldments of each super-austenitic stainless steel. The employment of these welding techniques provided a full range of unmixed zone formation. The unmixed zone formation was metallographically evaluated using both optical light microscopy (OLM) and scanning electron microscopy (SEM) with varied etchants and etching techniques. The pitting corrosion behavior of the unmixed zone was evaluated using immersion corrosion tests in 10% FeCl3. Publication of this document - WRC Bulletin No. 428 was sponsored by the Welding Research Council, Inc.