Artificial active sites were studied during nucleate pool boiling of degassed carbon tetrachloride. The pressure of the system was controlled a t one and also at one and one-half atmospheres. Dynamic two phase behavior in a cylindrical cavity was investigated visually. A glass capillary was inserted through the heat transfer surface and the bore supported ebullition in the absence of naturally occurring nuclei. When ebullition occurred at the surface, a falling liquid film was observed on the cavity wall. Three cylindrical cavities with diameters ranging from 0.004 inches to 0.008 inches were formed in a stainless steel heat transfer surface. The surface superheat required for ebullition ranged from 1.5°F to 43.2°F. For an appreciable range of superheats, the cavities were armed with vapor but did not support ebullition. Bubble columns were photographed using a high-speed motion picture camera. The diameter of detaching bubbles increased with higher surface superheats. Bubbles generated from the glass capillary had lower frequencies than those generated from stainless steel heating surfaces. The liquid flow around a detaching bubble assumed two modes. Axially symmetric pulsating flow and flow by continuous rotation about an active site were recognized by their effects on bubble shape.