The formation of AcAn from PHX, nitric acid, and acetic anhydride has been investigated at various temperatures. The effects of added acetic acid and added salts have been determined. While the reaction is first order with respect to PHX, it is of an order 2.6 with respect to nitric acid. The reaction rate is significantly decreased by small concentrations of sodium nitrate. The reaction is also characterized by a secondary salt effect and a low apparent activation energy of about 2 kcal. mole−1. A mechanism has been suggested in qualitative, and to some extent quantitative, agreement with the experimental data. It is postulated that the conversion of PHX involves a rate-controlling ionic reaction between PHX and nitric acid, and that this is followed by a rapid acetylation of the acidic intermediate to AcAn.