A recent innovation in evacuated tubular solar collector technology is described. A desorbable gas, which specifically adsorbs on the graphitic solar selective surface, can reversibly degrade the vacuum by providing a heat conduction path at elevated temperatures. The stagnation temperature of the system is thus limited in a controllable manner without significantly degrading the low temperature performance. A simple theory incorporating the Langmuir adsorption isotherm and the Knudsen free molecule transport regime is used to describe the phenomenon.