An adapter of the invention for fitting an NMR sample tube having an outside diameter into an NMR spectrometer sample tube holder designed for a larger outside diameter sample tube includes a one-piece elongate cylinder having an upper portion and a lower portion. The lower portion has an outside diameter sized to fit within a preselected size NMR spectrometer sample tube holder. The cylinder has a hollow bore axially extending through the upper portion and the lower portion, the hollow bore having a preselected inside diameter sized to accept an NMR sample tube smaller than the preselected size NMR spectrometer sample tube holder.