Variable-temperature UV-vis and Raman spectra have enabled detection of a thermochromic phase transition for [(Bu2Sn)-Bu-n](n), with an onset temperature T-c at ca. 260 K. The exact T-c value depends on the sample prehistory and the rate of cooling. This transition proceeds over a temperature interval from ca. 260 to ca. 200 K, when it reaches completion, and it involves two modifications: one with a disordered polymer backbone, which corresponds to a UV band at ca. 390 nm and a broad Raman feature for the v(Sn-Sn) mode at ca. 120 cm(-1), and a more ordered one (apparently, all-anti or transoid),' to which a red-shifted UV band at ca. 420 nm and a very intense narrow Raman line for the v(Sn-Sn) mode at ca. 145 cm(-1) are assigned. No abrupt side chain ordering was observed at T-c, but a gradual ordering, involving rotational isomerism about the C-C bonds, occurs on further lowering of the temperature. Comparison is made with data obtained for other polystannanes [(R2Sn)-R-n](n) (R = n-hexyl, n-octyl).