Kaiser Aluminum & Chemical Corporation, in cooperation with Ravenswood Aluminum Corporation, conducted a program to determine the optimum Mg level in aluminum-silicon vacuum brazing filler alloys. Most users of vacuum brazing sheet operate at one extreme or the other, i.e., Mg levels around 0.2% or near 1.5%. Both alloy families have problems. We fabricated and brazed several brazing sheet materials with K319 cores (similar to AA3005) and 4045 type filler metals with Mg levels ranging from 0.2% up to 2.0%. The optimum magnesium level in Al-10Si filler metals typically used in vacuum brazing applications was determined to be in the range from 0.6% to 1.0%. Filler metal Mg levels outside the optimum range result in smaller fillet sizes. Empirical data suggest that for filler metal Mg levels about 1.00%, fillet sizes are about twice those for filler metals containing approximately 0.2% Mg. When the Mg level has been optimized around the 1.0% level, the fillet sizes are 25% to 50% larger than for the 1.5% Mg alloy. The mechanism responsible for the change in fillet size as a function of Mg level can be explained by examining the role of surface tension (wetting) on the joint formation. Magnesium is a very effective liquid-vapor surface tension (γLV) modifier of molten aluminum. When the Mg level is near 0.2%, γLV≈ 720 Dynes/cm. This relatively high surface tension does not allow the filler metal to easily flow into the joint or to thoroughly wet the surfaces. As the Mg level in the filler metal is increased, the surface tension continuously decreases. Within the optimum range, the surface tension ranges from γLV≈ 640 to 660 Dynes/cm. Above 1.25% Mg, γLV approaches 630 Dynes/cm. As γLV decreases, the filler metal wets the surfaces too well. The filler metal not only wets the joints, but the surrounding surfaces as well. This results in smoother, but smaller joints.