Evaporation in solar thermal collectors normally takes place when the collector pump is not running—the so-called full stagnation. But it is possible that part of the heat transfer fluid evaporates inside a solar thermal collector field although the pump is operating and the collector field outlet temperature is significantly below the evaporation temperature. This operating status is called partial stagnation since only parts of the collector are affected by evaporation. Partial stagnation happens at a pronounced nonuniform temperature distribution in combination with a low mass flow rate and/or a high temperature level. A main reason for an irregular temperature distribution is a nonuniform flow distribution inside the solar thermal system. The paper presents an experimental investigation that analyzes the reasons and effects of partial stagnation occurrences. For this, outdoor measurements were made with a direct-flow vacuum tube collector. Criteria that promote partial stagnation have been identified, such as a coaxial tube design, a low system pressure, and a high gas content of the fluid. Performance measurements show no efficiency reduction during partial stagnation in the system investigated at a horizontal or positive collector slope. A high degree of partial stagnation, however, might pass into a complete evaporation of the collector volume although the collector pump is still running. This could lead to a complete blockage of the flow and a high thermal load of the system components. In all cases, partial stagnation leads to an unstable operation and a high load of the collector fluid and should, therefore, be avoided by design measures. A minimized risk for evaporation during operation is achieved by a more equal flow distribution inside the collector and the whole collector field, air bubbles, and solid particles should be completely removed. In addition, the gas content dissolved in the fluid may be reduced and the system pressure level may be increased in order to raise the boiling temperature.