[en] Highlights: • The reaction of the [MoO₄]²⁻ monomer with polyoxomolybdates is thermodynamically favorable. • The protonation degree of [Mo₅O₁₇]⁴⁻ and [Mo₇O₂₄]⁶⁻ defines the pathway to form larger polyoxomolybdates. • The favourable reactions involve the consumption of acid and release of water. • The main mechanism to form larger polyoxomolybdates is through the [MoO₄]²⁻ reaction with smaller polyoxomolybdates. Polyoxomolybdates have been intensely investigated, but their mechanisms of formation are not completely understood. The complex equilibrium of different species is affected by concentration, pH, ionic strength and temperature. It is a challenging system to model using computational chemistry. In the present work, density functional calculations were carried out using the polarizable continuum method to include solvent effects in an effort to provide insight into the mechanism of polyoxomolybdate formation in aqueous solution. We establish a possible sequence of reactions for the formation of small polyoxomolybdates containing up to 8 Mo by addition of the monomeric unit [MoO₄]²⁻.