An analysis is given for a method of measuring the bulk modulus of a liquid by means of a Helmholtz resonator driven by an oscillating diaphragm in the cavity wall. The influence of the pressure transducer on the resonant frequencies is considered. In addition to the bulk modulus, the viscosity can be estimated by measuring the “Q” of the cavity.One resonator was built, tested, and calibrated. Some difficulty was encountered in measuring resonant frequencies of the fluid‐filled cavity at atmospheric pressure because of the formation of vapor bubbles. The results obtained for five common liquids compared favorably with earlier measurements of bulk modulus found in the literature.This method is especially suitable for measuring the bulk modulus of corrosive liquids since the liquid may be completely sealed in the cavity under pressure and no measuring probes are moved through the liquid.