Various isoreticular monometallic (Co2+, Ni2+, Cu2+, and Zn2+) and bimetallic (Co-Ni, Co-Zn, Mn-Ni) trimesate MOFs have been prepared by a fast (10 min) and green synthesis method from aqueous solutions, at room temperature and ambient pressure. A combined XRD and SEM/EDX analysis clearly revealed bimetallic compounds form true solid solutions rather than a simple physical mixture of pure-phase monometallic compounds. Moreover, a detailed evaluation of the evolution of cell parameters with the composition provides strong evidence indicating a preferential occupation of one crystallographic position (bidentate terminal sites) by Co2+ (or Mn2+) ions. This leads to a precise and predictable array of metal ions in the framework, which can be finely tuned by changing the overall composition of the bimetallic MOF. Implications are envisaged in the design and catalytic properties of well-defined single-site catalysts. © 2019 American Chemical Society.