Despite the vast number of polyoxometalate clusters now known, an ongoing and important challenge is to understand causality in the assembly of “complex” clusters at a mechanistic level, since this is the only way the rational, targeted synthesis of new compounds will ever be achieved. Often, the complexity of the reactions themselves makes such investigations near impossible, as very small changes can often make dramatic differences. Herein, we explore a very simple A + B binary synthetic system that gives rise to the facile assembly of two isomeric anions, FeIII(H2O)2{γ-FeIIISiW9O34(H2O)}211– (1) and FeIII(H2O)2{γ-FeIII 2SiW8O33(H2O)2}­{γ-SiW10O35}11– (2), which can be formed as individual and dimeric species (3) and (4). Furthermore, the simple binary nature of this synthetic system allowed its investigation by a comprehensive time-resolved ESI-MS analysis, yielding unprecedented mechanistic information regarding the initial interactions and reorganizations of the {γ-SiW10} precursor in the presence of Fe2+.