To determine the supercritical reaction conditions of anisole acylation with acetic anhydride using certain solvents such as carbon dioxide or propane, the critical temperatures and pressures of the nominal reacting mixture ( anisole + acetic anhydride + p-methoxyacetophenone + acetic acid + carbon dioxide or propane) were measured with the composition representing the extent of reaction ( conversion of acetic anhydride) by using a high-pressure viewing cell. The effects of the initial composition, the addition of suitable solvent, and the extent of reaction on the critical properties of the reacting mixture were then investigated. Such information may be helpful to those who intend to carry out the acylation of anisole with acetic anhydride under supercritical conditions to improve the efficiency of current processes in the gas or liquid phase.