The partial hydrolysis of Al 3+ solution by titrating with Cr 3+ salt dissolved in KOH solution formed polycationic Keggin oligomer Al 25.7 Cr 6.3 S 2 16+ (Al 25.7 Cr 6.3 O 8 (OH) 60 (H 2 O) 28 (SO 4 ) 2 ). • Synthesis, crystallization, and chemical characterization of a novel Cr 3+ substituted polycationic Al 3+ Keggin-type oligomer. • Elucidating structure of this Keggin oligomer enhanced the atomistic level understanding of Cr 3+ incorporation in this system. • Formation of this novel Keggin oligomer pointed out important role base cation may play in oligomerization process. Keggin-type aluminum polycations are an important class of compounds from metal-oxo family that includes polyoxometalates and other nanocomplexes. Investigation of the Al 3+ based polycation system is of particular importance given the environmental relevance within acid mine drainage and application in wastewater treatment. Within streams impacted with acid mine drainage, different toxic metal species, including chromium, can incorporate into these soluble polycations and impact the transport of contaminants. Currently, there is a limited understanding about chemical interaction of chromium with Keggin-type aluminum polycations; therefore, the work herein describes the synthesis and characterization Cr 3+ substitution within Keggin-based polyaluminum species. The polyaluminum cations crystallized as [Al 25.7 Cr 6.3 O 8 (OH) 60 (H 2 O) 28 (SO 4 ) 2 ](SO 4 ) 8 (H 2 O) 29 from a partially hydrolyzed Al 3+ solution and structural elucidation obtained by single crystal X-ray diffraction revealed the presence of Cr 3+ within the octahedral sites of the Keggin species. Inductively coupled plasma mass spectroscopy confirmed the presence of Cr 3+ in the structure with Al 3+ /Cr 3+ similar to refined crystal structure. Additional physicochemical characterization, including vibrational spectroscopy, thermogravimetry and powder X-ray diffraction were performed on the isolated crystals and confirm the structural model of the solid phase.