The considerable attention that has been given to the polarographic behavior of cyclodextrins and sonic of their complexes but not to those with arenediazonium ions, ArN2+, together with the growing biochemical importance of ArN2+ due to their ability to generate aryl radicals, prompted us to analyze the effects of beta-cyclodextrin, P-CD, on a model arenediazonium salt,p-nitrobenzenediazonium (PNBD) tetrafluoroborate in aqueous acid solution. Spectrophotometric and electrochemical kinetic data show that at pH < 5, observed rate constants, k(obs), increase upon increasing [beta-CD] up to 1345 times its value in pure water when [beta-CD] = 40 [PNBD]. HPLC analysis of reaction products shows that large amounts of the reduction product nitrobenzene, PNBH, are formed at the expense of p-nitrophenol, PNBOH, suggesting that beta-CD promotes homolytic dediazoniation. Quantitative conversion to products is achieved at any [beta-CD] when pH < 5. The S-shaped product distribution observed at higher pH was interpreted in terms of the behavior of PNBD as a Lewis acid, yielding a pK(A) = 6.34, in perfect agreement with that obtained electrochemically (6.45) and with published results. The P-CD-induced change in the dediazoniation mechanism of PNBD can be rationalized on the basis of the effects of beta-CD on its electrochemical behavior. Addition of beta-CD to an aqueous acid PNBD solution causes a substantial decrease in peak currents, i(p), and shifts in opposite directions for the apparent peak potential values, E-app, of the -N-2(+) and -NO2 groups. beta-CD hinders the reduction of the nitro group since E-app (-NO2) is shifted toward more negative values, in contrast with the E,,p, shift observed for the one-electron reduction of the -N-2(+) group to form the aryldiazenyl radical, ArN2., which is shifted toward more positive values, thus favoring the reduction of PNBD. Quantitative analysis of the Eapp and ip values allowed estimations of the binding constant of ArN2+ and, for the first time, of the binding constant of ArN2. (or Ar-.) with beta-CD, which is much higher than that of the parent ArN2+. Either the decrease in peak currents or E,,, shifts are interpreted in terms of the formation of a 1:1 inclusion complex between beta-CD and PNBD, with the -NO2 group inserted into the CD cavity. The specific spatial configuration of complexed PNBD ions allows solvation of the positive charge of the -N-2(.) group mainly by the secondary hydroxyl groups of beta-CD, favoring further reaction to yield a highly unstable transient intermediate, a Z-diazo ether, which was detected experimentally. Further evidence for the formation of such a transient intermediate was obtained from analysis of the kinetic data, which show that k(obs) increases upon increasing W-CD] following a saturation kinetics pattern, All evidence is consistent with two competitive reaction pathways, the thermal D-N + A(N) decomposition of ArN2+ and the rate-limiting cleavage of a transient diazo ether "complex" formed in two rapid pro-equilibrium steps involving the formation of a PNBD-beta-CD inclusion complex followed by the formation of the transient intermediate.