An improved procedure has been developed to prepare RCOOPO2-Ado (R = C6H13 and C15H31), the intermediate in the enzymatic synthesis of acyl-CoA's. The product has been characterized by spectral methods which, in turn, were used to define the hydrolysis rates of RCO-AMP. When R is a fatty acyl group, the hydrolysis rate is 10-fold slower than when R is aminoacyl. In both cases, the hydrolysis rate is enhanced by Mg2+. We speculate that the rate acceleration is due to intramolecular participation of the second carbonyl group in the aminoacyl residue.