Crystalline Cs+(18-crown-6)(2)e(-) was found to be a linear chain Heisenberg antiferromagnet (J/k(B) = -38.3 K). It undergoes a slow, irreversible transition above similar to 230 K from a crystalline low temperature (LT) phase to a disordered Curie-Weiss paramagnetic high temperature (HT) phase. A 100 ppm diamagnetic (more shielded shift of the Cs-133 MAS NMR peak accompanies this transition. EPR studies confirm the transition and are in accord with the conversion of the known crystal structure to a disordered phase. This HT phase undergoes an additional first-order reversible transition upon cooling below similar to 220 K (Delta H = 0.33 kJ mol(-1) upon rewarming) accompanied by the formation of two peaks in the Cs-133 NMR spectrum. Variable-temperature powder X-ray diffraction confirms the disordered or microcrystalline nature of the HT phase(s).