Abstract Though great efforts have devoted to prepare planar Al13 [Al13(OH)24(H2O)2415+, M-Al13], the lack of knowledge of its stability and hydrolysis behavior in aqueous solution limits its practical application as coagulant. In this study, the stability, morphological transformation and coagulation property of M-Al13 were thoroughly investigated. The 27Al-NMR spectra revealed temperature-dependent morphological transformation from M-Al13 to K-Al13 in aqueous solution. This transformation was completely inhabited below 5 °C, but was accelerated with increased temperature from 5 to 70 °C. Moreover, higher temperature and longer aging time led to further transformation of K-Al13 to the aluminum species with higher degree of polymerization such as Al30. Thus, low temperature was favorable for the morphological stability of M-Al13. Due to the maintenance of M-Al13 structure, all as-prepared M-Al13 crystal, M-Al13 solution (0 °C) and freeze dried M-Al13 showed excellent and comparable flocculation property in the terms of floc growth rate, floc size and floc strength. Interestingly, the flocculability dramatically decreased after M-Al13 solution aged at 25 °C or 40 °C for 120 min with 29% and 62% of M-Al13 transformed to K-Al13, respectively. These results implied that M-Al13 was an important flocculant with better performance than the well-known and efficient flocculant K-Al13. Our study about the stability and hydrolysis behavior of M-Al13 might prompt its application as a highly efficient flocculent for water purification.