It is shown by a diffraction technique that 2‐naphthalenethiol (C10H8S) undergoes a first‐order phase transition which occurs at 313 ± 0.5 K. The low‐temperature form III is ordered whereas the high‐temperature one I is disordered and may be kept as metastable far below 313 K. Thermal expansion studies, performed on the stable form III and on both metastable and stable forms I, indicate that: form III exhibits a pronounced `pretransitional' effect; discontinuities of the lattice constants occur at the III →I transition; the lattice‐constant thermal‐expansion curves of form I do not show any inflexion at the metastable to stable transition; in every case a large thermal expansion anisotropy is observed. Curiously, the compacity of the ordered low‐temperature form is lower than that of the disordered high‐temperature form. The JCPDS Diffraction File No. of the low‐tempera