The Solar Probe Plus (SPP) spacecraft will fly further into the Sun’s corona than any previous mission, reaching a minimum perihelion at 9.5 solar radii from the center of the Sun. The solar arrays powering the spacecraft will operate under unusually high irradiances and temperatures. The array design, material choices, and necessary test facilities for SPP are therefore quite different from those used on traditional space panels. This paper gives an overview of the high-irradiance high- temperature vacuum (HIHT-Vac) reliability testing completed to date at EMCORE on three small-scale coupons representing two competing SPParray technologies. Both technologies successfully passed the HIHT-Vac test with no measurable performance, visual or mechanical degradation, reaching a key milestone in the development of the SPP array.