<p>In this work, niobium-doped phosphomolybdic acids with the general formulae H_3+nPMo_12-nNb_nO₄₀ (n = 0, 1, 2, and 3) were prepared, characterized, and evaluated as catalysts in the acetalization of the β-citronellal with alcohols. The impact of niobium load on the structural and catalytic properties of phosphomolybdic acids was assessed. Among the niobium-substituted phosphomolybdic acids, the H₄PMo₁₁Nb₁O₄₀ acid was the most active and selective toward the methyl acetal β-citronellal, achieving a TON 196, TOF of 0.11&#xa0;mmol/s, and 98% yield after 30&#xa0;min reaction. The effects of main reaction variables, such as time, temperature, catalyst load, type of alcohol, and niobium load, on the conversion and selectivity of the reactions were assessed. The highest activity of H₄PMo₁₁Nb₁O₄₀ was attributed to the adequate proportion of H⁺ and Nb⁵⁺ cations. The H₄PMo₁₁Nb₁O₄₀ catalyst was easily recovered and reused without loss of activity.</p> Graphical Abstract <p></p>