the reason for the difficulties in obtaining the rateconstants published in ref 4 from the raw data of ref 2 stemsfrom the (partial) mislabeling of the columns in the thesis ofG. C. Alexander (ref 2). We confirm that, when interpreted assuggested in the Comment (i.e., that in Appendix C of ref 2 thecolumn for the weight of maleic anhydride (MA) is labeled asthe weight of methylcyclohexene dicarboxylic anhydride(MCDA), the reaction product, and the column for the weightof MCDA is labeled as the weight of MA, with the exceptionof the two last rows where the numerical information corre-sponds to the column headings), the rate data from the ref 4are reproduced.The major point of our paper (ref 1) is that the measurementsin the earlier studies (refs 2 and 4) were performed in amultiphase system. The Comment erroneously implies that thisconclusion is based on (or is supported by) the observation thatthe apparent rate constant observed in our experiments in themultiphase region is larger than the true rate constant in thesupercritical phase measured at very low mole fractions of thereactants. Throughout our paper (ref 1) we compare our resultswith both the rate constants reported in ref 1 and the rateconstants obtained from the raw data of ref 2 based on theoriginal headings of the tabulated data. In both cases a significantdiscrepancy between our measurements and the previousmeasurements exists. However, our conclusions were not madeon the basis of the relative sign of this discrepancy, as could bederived from the Comment. On the contrary, several briefdiscussions are offered (ref 1) on the difficulties in theassessment of the impact of the second phase on the overallreaction rate, on the adequacy of the sampling procedure, andsubsequently, on the measured apparent rate constant.“The presence of a second (liquid) phase could affect thereaction rate in several ways. Both acceleration and decelerationof the overall reaction are possible, depending on both thereaction rate constants in the two phases and their compositions.If the liquid phase is enriched by both reactants, then theacceleration of the overall reaction is anticipated even for equalintrinsic reaction rate constants in both phases. If the reactantsare partitioned between the two phases, then the decelerationof the overall reaction could be expected. And, finally, the rateconstants in the supercritical and the liquid phases could differdue to the solvent effects.” (Reference 1, p 9428).“An additional reason for the difference in the apparent rateconstants measured in two-phase systems could be in thedifference in experimental procedures used. In the current work,the mole fractions of isoprene in the samples obtained fromthe gas phase were measured (Figure 3b). In refs 6, 7, bothmaleic anhydride (MA) and MCDA were sampled from the gasphase, but only the ratios of the mole fractions of MA andMCDA in the samples are available.