Potassium caprate, CH3(CH2)(8)CO2K, having 99% purity was synthesized, and its thermal structural transition behavior was investigated by vibrational spectra. A normal-mode analysis was made for the crystalline phase I (A-form) at room temperature, and vibrational assignments were made. As temperature increased from phase I to phases II and III, the partial melting of the alkyl chains was observed. At the same transitions, the occurrence of the conformational disorder around the carboxylate end was suggested. The end effect on the vibrational frequencies in the methylene progressive region was also discussed.