Few studies have explored the change in thermal stability of poly(oxypropylene) (POP) ammonium ions after intercalation, even though several studies have focused on the modification of montmorillonite (Mt) with POP amine hydrochloride. The purpose of the present study was to understand the effect of chain length of POP amine hydrochlorides on the basal spacing of modified Mt, and the amount and thermal stability of the ammonium ions intercalated. The relations between basal spacing, organic fraction, and thermal stability of the ammonium ions intercalated were also explored. Series of modified Mt were prepared via ion-exchange between Na-montmorillonite (Na+-Mt) and POP diammonium ions or POP triammonium ions with different chain lengths, and were then characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, and simultaneous differential scanning calorimetry-thermogravimetric analysis. The results revealed that the basal spacing of modified Mt increased with the hydrophobic chain length of the POP ammonium ions. The amount of triammonium ions intercalated was close to the theoretical amount, while the organic fraction of modified Mt was directly proportional to the basal spacing of modified Mt. The intercalated ammonium ions were, therefore, contained within the interlayer space of Mt. After intercalation, the thermal stability of the POP ammonium ions with various chain lengths was reduced; i.e. Tonset was reduced by 7–60ºC for short-chain POP ammonium ions (D400 and T403) and by 177–192ºC for long-chain ions (D2000, D4000, T3000, and T5000).