In order to supplement dynamic measurements in shear and bending, the vibration tester of the Franklin Institute Laboratories was used to measure the bulk compressibility of plastics using harmonic vibrations. Polyethylene and plasticized polyvinyl chloride were investigated. The values of the compressibility were comparable to the static values as far as they are known. A noticeable phase shift between pressure changes and volume changes could not be detected with certainty so that the bulk modulusBin dynes/cm2is not a complex quantity. Especially for the polyethylene, a temperature range from −25 to +95°C and a frequency range from 0.0003 to 5 cps was used. The isothermals obtained could be shifted together using the method of reduced variables of Ferry. From the frequency shiftsaT, an activation energy of 60 kcal/mole was obtained.