A gravimetric analysis apparatus to measure the mass-transfer rates of solid solutes into supercritical solvents has been designed, built, and tested. The heart of the apparatus is a miniature thin film strain gauge load cell which measures the mass of a solid suspended in a flowing fluid continuously over the duration of the experiment. From knowledge of the geometry of the solute pellet, the mass flux at the surface interface is determined directly. In this work, mass flux data was collected for the dissolution of naphthalene into supercritical carbon dioxide at 35 and 55 degrees C and at pressures ranging from 8359 to 12 000 kPa. From the data, binary diffusion coefficients were calculated and compared with published values to verify the methodology. As the calculated values fall within the range of the published diffusion coefficients, the apparatus can be considered to have been verified successfully and is fully operational.