Bismuth ferrites have been prepared using a hydrothermal method in a reaction solution containing a mineralizer with various ratios of KOH/NaOH. The phases and morphology of the bismuth ferrites were examined for reaction systems with two different molar ratios of Bi³⁺/Fe³⁺, 1 and 1/2. For the reaction system of Bi³⁺/Fe³⁺ = 1, we were able to tune the mean thickness of Bi₂Fe₄O₉ plates by varying the ratios of KOH/NaOH. For the reaction system of Bi³⁺/Fe³⁺ = 1/2, the main phase transferred from mixed BiFeO₃ and Fe₂O₃ to Bi₂Fe₄O₉ when the ratio of KOH/NaOH decreased to less than 1. The morphology of Bi₂Fe₄O₉, in this case, is more rectangular and block-like with dominating {001} facets. The relatively faster kinetics of the Bi₂Fe₄O₉ formation triggered by the presence of sodium ions determines the available concentration of Fe³⁺ ions for further growth which, in turn, affects the mean thickness of Bi₂Fe₄O₉ plates. The energy bandgaps of Bi₂Fe₄O₉ plates, which increase from 2.18 eV to 2.27 eV with decreasing thicknesses, lie in the range of visible-light spectra and are suitable for the application of visible-light photocatalyst.