The solvent-bridged caesium phosphide [CsPH(eta6-2,4,6-(t)Bu3C6H2)]2(eta3-toluene)0.5x, catena-[(micro-eta3-toluene)-bis[caesium(2,4,6-tri-tert-butylphenylphosphide)]], undergoes a reversible solid-state, order-disorder phase transition characterized by the doubling of the unit-cell volume at low temperature achieved by doubling one unit-cell vector. The unit-cell parameters at 293 (2) K (form A) are: a = 11.147 (4), b = 14.615 (4), c = 14.806 (5) A, alpha = 70.57 (3), beta = 71.85 (3), gamma = 72.93 (2)degrees, V= 2112.5 (12) A3, Z = 2, rho(calc) = 1.362 g cm(-3), R1 = 0.0513 for 5462 reflections, wR2 = 0.0947 for all data. The unit-cell parameters at 173 (2) K (form B) are: a = 14.6241 (3), b = 14.7393 (3), c = 22.0720 (4) A, alpha = 72.2117 (7), beta = 73.3659 (8), gamma = 70.2953 (7)degrees, V = 4174.8 (2) A3, Z = 4, rho(calc) = 1.379 g cm(-3), R1 = 0.0405 for 14,010 reflections, wR2 = 0.1326 for all data. With a minor change, the key structural features discussed previously for form A [Rabe et aL (1998). Inorg. Chem. 37, 4235-4245] remain unchanged. The eta3-toluene ligand is observed to be disordered at 293 (2) K and ordered at 173 (2) K, with the order-disorder phase transition occurring at approximately 278 (2) K.