Phase equilibria data for the KHSO4-H2SO4-H2O ternary system and the KHSO4-CrO3-H2SO4-H2O quaternary system at 313.15 K were measured, and the phase diagrams were constructed. Furthermore, analyses and discussions were made on the crystallization zones in the phase diagrams. All results can offer fundamental basis for preparation and purification of chromium trioxide by reacting potassium dichromate with sulfuric acid.