Communication: Purification of single-shell nanotubes is essential if reliable information about their physical and chemical properties is to be obtained. Using the present synthetic techniques, samples typically contain 30% or more amorphous carbon and catalytic metal particle impurities. An extremely simple and efficient method of purification, involving boiling the nanotubes in concentrated nitric acid, is reported. It is demonstrated that this method removes most of the impurities and, unlike some other methods, is not sensitive to the quality of the starting material.