Surface tension of a single liquid or interfacial tension between two liquids can be measured by permitting the denser liquid to drain from a reservoir through a vertical tube of diameter about 8 mm which acts as a siphon. As the drainage rate decreases, the meniscus at the base of the drainage tube becomes unstable and thus the siphon effect ceases. The minimum level of liquid in the reservoir during this incomplete drainage process provides an indication of the surface or interfacial tension of the draining liquid. This concept has been tested for sixteen liquids in air and for four liquid-liquid systems.