The molecular sieve UTD-1 is investigated using scanning and transmission electron microscopies (TEM), solid-state NMR spectroscopy, electron (ED) and X-ray diffraction (XRD), adsorption studies, and catalytic test reactions. The results confirm that UTD-1 is the first high-silica zeolite to contain a one;dimensional, extra-large 14-ring pore system. TEM and ED show that UTD-1 is faulted along the (002) planes. Simulations of XRD patterns of faulted structures using DIFFaX indicate that the XRD pattern of a framework containing the so-called double crankshaft chains is in better agreement with the experimental pattern than a framework with the narsarsukite chains previously reported. Thermal/hydrothermal stability studies show that UTD-1 has similar stability to other medium- and large-pore, high-silica zeolites. The ratio of isomerization to disproportionation, and the distribution of trimethylbenzene isomers in the m-xylene isomerization test reaction from UTD-1 are similar to those obtained from other large-pore zeolites (zeolites Y or L). However, UTD-1 shows a p-/o-xylene ratio of products below one.