The nature and level of the vacuum atmosphere together with the depth and composition of the oxide layer on brazing sheet can have a profound effect on the quality of the brazed joint. For example, magnesium is needed in the furnace atmosphere to obtain a good fillet. However, there is a lack of understanding of the interrelationship between oxide thickness and the amount of magnesium in the furnace atmosphere. This paper attempts to address this area. Samples of brazing sheet with modified oxide layers, some of which simulate storage or transport conditions were brazed with varying magnesium content in the furnace atmosphere. A correlation was made between these two parameters and the material's brazeability. The main conclusion of this study was that if sufficient magnesium is present in the furnace atmosphere the material can tolerate significant changes in oxide thickness and type without sacrificing brazeability.