Two new volumetric sorption techniques have been developed to measure thermodynamic and mass-transport properties in polymer-solvent systems at elevated pressures. They are both variations of the pressure-decay technique and differ in how the initial gas density is measured. In the first and simplest variation, the initial density is measured by extrapolation of the mass uptake curve from the region of usable data to the experiment starting time. In the second variation, the initial density is measured gravimetrically using a small titanium capsule. Concern has been voiced as to whether the extrapolation-type experiment is capable of measuring the initial gas density as accurately as a dual-chamber technique. To evaluate the accuracy of the extrapolation technique, solubility and diffusion data were collected for carbon dioxide, ethylene, and nitrogen in low-density polyethylene (LDPE) using both methods. The results indicate that the simpler extrapolation method produces the same diffusivity and solubility results as the more complicated dual-chamber method.