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The thermal denaturation ultraviolet absorption spectra of N6,N6-dimetliyladenyly1(3'+ 5')-N',N'-dimethyladenosine (m:ApmzA), which is a common sequence in 16 -18-S ribosomal RNA, in aqueous buffer at pH 7 have been measured over the temperature range 3 -90 "C. These data have been used to determine the thermodynamic quantities associated with the intramolecular stacking equilibria. At 25 "C in neutral aqueous solution m:Apm:A exists mainly (about 81 '%;) as a stacked form, so that the stacking interactions are stronger than those in the parent unmethylated adenylyl-(3'-5')adenosine (ApA), where about 52% is stacked. From the parameters of A H and AS, it is concluded that 'hidden' hydrophobic interactions are of prime importance in the enhanced stability of m:Apm:A.Transphosphorylation reaction of ApA and m6,Apm:A to form the corresponding cyclic 2', 3'-phosphates has been studied. First-order rate constants at 25 "C for the reactions, which are basecatalyzed, have been obtained. Insertion of two methyl groups at N-6 of ApA reduces the rate of transphosphorylation. Effects of stacking on rates are discussed in the light of reaction mechanisms.