The effects of temperature, pressure, and atmosphere composition were examined to determine how each influenced vacuum brazed aluminum radiator tube-to-header joints. Brazing at 1 x 10-5 Torr (1.3 x 10-3 Pa), while varying the braze temperature from 1070°F (577°C) to 1125°F (607°C), showed that acceptable joints were produced in the 1080°F (582°C) to 1110°F (590°C) range. Below 1080°F (582°C), insufficient melting caused poor filleting. Above 1110°F (590°C) severe tube core dissolution occurred. Increasing the pressure (constant temperature) by introducing air into the furnace caused extremely poor fillets above 2.0 x 10-4 Torr (3 x 10-2 Pa). Acceptable fillets were formed in N2 saturated with 10,000 ppm of H2O at 9 x 10-4 Torr (0.12 Pa). XPS analyses of the air generated oxides showed that they were 700 A thick or less, and composed of MgO and Al2O3. Techniques were developed which allow the operating temperature and pressure of the furnace to be estimated from brazed radiators.