Abstract: A pycnometer apparatus is disclosed for accurately determining the volume of a condensed material sample. Equal free space volumes are established in a pair of gas filled chambers before and after insertion of a sample of the condensed material within one of the chambers. The volume of the sample receiving chamber is increased to maintain equal free space volumes after the sample is inserted, the adjusted increase in volume providing a highly accurate determination of the volume of the sample. The apparatus may also be employed to determine the compressibility or the porosity of a material sample.