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<strong class="journal-contentHeaderColor">Abstract.</strong> Atmospheric aerosols, especially internally mixed organic-inorganic aerosols, exhibit complex phase behaviors that affect their size evolution, optical properties, and chemical reactivity, ultimately impacting climate and human health. Although parameterizations for secondary organic aerosol phase state exist, predictive models based on primary predictors for efflorescence in organic-inorganic aerosols remain underdeveloped. In this study, we evaluated several chemical parameters, including equivalent O:C ratio, organic mass fractions, glass transition temperature (T<sub>g</sub>), and viscosity (η), and identified aerosol viscosity as the primary predictor of efflorescence relative humidity (ERH) in internally mixed organic-inorganic aerosols. We developed a linear viscosity-ERH model based on ERH and log<sub>10</sub>(η), which defines the boundary conditions for aerosol efflorescence when η< 4.76 × 10² Pa·s. Additionally, we showed that efflorescence is inhibited when η> 4.76 × 10² Pa·s. Validation using an independent dataset showed strong agreement between predicted and experimentally measured ERH (R² = 0.95). A multivariate regression model incorporating η and T<sub>g</sub> improved prediction accuracy but was limited by T<sub>g</sub> parameterization for complex organic-inorganic mixtures. Our findings highlight the role of aerosol viscosity in controlling efflorescence and emphasize the need to develop improved aerosol viscosity measurement techniques to better constrain aerosol phase transitions in atmospheric models.