A kinetic study of the hydrolysis of 39.8 wt.‐% acetyl cellulose acetate has been made as a function of pH and temperature over the pH range of 2.2–10 and temperature range of 23–95° C. The hydrolysis reaction was carried out on highly porous membranes under quasihomogeneous conditions and the data have been treated as a pseudo‐first‐order reaction in acetyl concentration. The reaction can be represented by the equation article empty k_1\rm=\;k_\rmH^+\left\rmH^+\right\rm+k_\rmOH^-\left\rmOH^-\right+k_\rmH_2O, and where article empty k_\rmH^+\rm=5\rm.24\rmx10^\rm5\rmexp\left{\rm‐16\rm.4x10^\rm3/RT\right\},\rmk_\rmOH^‐\rm=1\rm.55\rmx10^\rm4\rmexp\left{\rm‐8\rm.1x10^\rm3/RT\right\}, and article empty k_\rmH_2O=4.25\;*10^-2\rmexp\left{-11.5*10^3/RT\right\} (where the quantities in brackets are activities of the ions shown).