An apparatus for measuring dissolution rates of solids in the form of disks was designed to possess the following features. The solvent flowed continuously and reproducibly past the disk at various rates associated with calculable Reynolds numbers, Re. The effluent solution was adequately mixed before analysis. The concentration of dissolved solute was much less than the solubility. The surface area of the disk in contact with the solvent was constant during measurements. The dissolution rate of the disk was reproducible, and the disk and its surface could be readily characterized. The apparatus was tested at 37 degrees with compressed potassium chloride and water. The intrinsic dissolution rate, G, was a linear function of Re from Re=360 to greater than 6000. This relationship enabled one unknown constant in each dissolution theory to be expressed in terms of Re. For the diffusion layer model, the thickness of this layer, calculated from the experimental value of G, agreed well with that calculated from the various physical properties, provided that natural convection did not predominate. The dissolution of potassium chloride in this system was, therefore, controlled by diffusion.