In2Se3 nanowires synthesized by the VLS technique are transformed by solid-state reaction with copper into high-quality single-crystalline CuInSe2 nanowires. The process is studied by in situ transmission electron microscopy. The transformation temperature exhibits a surprising anisotropy, with In2Se3 nanowires grown along their [0001] direction transforming at a surprisingly low temperature of 225 degrees C, White nanowires in a [1120] orientation require a much higher temperature of 585 degrees C. These results offer a route to the synthesis of CuInSe2 nanowires at a relatively low temperature as well as insight into the details, of a transformation commonly used in-the fabrication of thin-film solar cells.