The acetic acid-ammonium acetate methods of catalyst rejuvenation by lead removal have been of considerable interest. The effectiveness of these methods was therefore investigated in the laboratory as a function of temperature, duration of treatment, type of agitation, and cation, using mainly catalyst samples poisoned by treatment with a lead acetate solution, with confirmation by vehicle-aged catalyst samples. The results have, in general, been encouraging. For instance, washing of poisoned catalyst samples with 10 wt % acetic acid solution at room temperature for half an hour was sufficient to improve the hydrocarbon conversion of a catalyst poisoned with 5 wt % Pb (typical of a catalyst aged for about 50,000 miles with fuel containing 0.03 g Pb/gl) by 15%. However, we found that such a rejuvenated catalyst, if subjected to a laboratory repoisoning test with 0.03 g Pb/gl fuel, loses hydrocarbon activity rapidly. The rate of activity loss upon repoisoning appears to be about 6 times faster than the average loss rate experienced before the rejuvenation treatment. On the other hand, a poisoned catalyst sample which was rejuvenated by a more drastic treatment (e.g. two treatments for 3 hrs each, at 195°F with ultrasonic agitation), appeared to have the same rate of activity loss upon further aging as a fresh catalyst after the initial aging. This study also showed that there is no significant difference in rejuvenation effectiveness among acetic acid or ammonium acetate at the same concentration.