To address the need for non-intrusive sensors for rocket plume properties, we propose a laser-based velocity diagnostic that does not require seeding, works in high or low temperature flows, and can be used over a broad range of velocities. Hydroxyl tagging velocimetry (HTV) "writes" a line of OH molecules into the flow and interrogates them after a short delay to determine the velocity. Since the markers are molecules, the method measures the gas velocity directly, avoiding errors due to particle drag associated with seeded techniques. The only requirement is that H2O molecules must be present in the flow, which is easily met by most rocket plume applications. Because OH molecules survive at high temperatures for appreciable lifetimes, it is anticipated that the HTV technique will work in even the highest temperature rocket plumes. The proposed diagnostic will provide measurements not obtainable by current methods, and will enable experimental data that can be used for validating computer models for rocket engine performance predictions.