The selective synthesis and in situ characterization of aqueous Alcontaining clusters is a long-standing challenge. We report a newly developed integrated platform that combines (i) a selective, atomeconomical, step-economical, scalable synthesis of Al-containing nanoclusters in water via precision electrolysis with strict pH control and (ii) an improved femtosecond stimulated Raman spectroscopic method covering a broad spectral range of ca. T he importance of Al (aluminum) in the biosphere and to human civilization is enormous. The scale of mining and production of Al compounds is second only to that of Fe (iron). Our lives are influenced by its use in electronics (1, 2), cooking and eating utensils, and food packaging, and as structural materials in the construction, automotive, and aircraft industries. Its deposition and migration as a mineral ore are controlled by its aqueous chemistry and speciation. Millions of tons of Al compounds are used worldwide each year for water treatment, and it is found in all drinking water (3). The behavior of Al in water plays significant roles in soil chemistry and plant growth (4, 5), for example, governing Al bioavailability, toxicity, and its overall impact in aquatic ecosystems (6). Meanwhile, aqueous Al clusters are gaining importance as solution precursors for the large-area deposition of Al 2 O 3 coatings with broad technological applications (7,8).Despite more than a century of study (9, 10), the complete portrait of aqueous Al chemistry remains unclear. Studies of aqueous Al chemistry are notoriously difficult because of the variety and complexity of the species that can be formed, encompassing monomeric, oligomeric, and polymeric hydroxides (11-17); colloidal solutions and gels; and precipitates. Synthesis is complicated by the fact that the counter-ions and the method and rate of pH change all have dramatic effects on product formation (18,19). Few methods exist for the in situ determination and assignment of molecular-level structures. For instance, 27 Al NMR can only identify certain Al aqueous species (15). Furthermore, unlike organic compounds, systematic spectroscopic signatures of metal hydroxide clusters are less accessible, making interpretation of experimental spectra challenging. We hereby report a combined synthesis, experiment, and theory platform for the study of aqueous metal clusters. Electrolysis is exploited to control the solution pH and counter-ion content precisely during cluster synthesis without using chemical reagents. The evolution of solution species is followed in situ by an improved femtosecond stimulated Raman (FSR) technique (20-22) that can detect weak signals associated with structure-defining vibrational modes. The resulting pHdependent Raman spectra are interpreted by juxtaposition to quantum mechanically computed vibrational modes to assign specific molecular structures. Through this integrated approach, we have discovered a speciation behavior for Al in water that has not previously been observed. We focus here on the synthesis an...