We previously established that zirconia is predominantly formed through an E1 elimination, with a small contribution from S N 1 substitution. As a mechanism, we thus combine an E1 elimination reaction with a condensation and an auto-catalytic step (Figure ). In this mechanism, zirconium isopropoxide is consumed through both the E1 elimination and the condensation steps. The equations are fully balanced and take into account the presence of ZrCl 4 as the reaction coproduct previously observed by P NMR. The concentration of ZrCl 2 (O i Pr) 2 in the presence of 11.4 eq. of TOPO at 340 • C was changed from 0.1, 0.2, and 0.3 mol/L (Figure ) and the differential rate equations were solved numerically by COPASI. A highly satisfactory fit was obtained in all three cases (Figure ). From the fitted rate constants, we conclude that the condensation step is fast with respect to the uncatalyzed E1 elimination. The auto-catalyzed E1 elimination is two orders of magnitude faster than the uncatalyzed first step. As the catalytic product, we hypothesize here ZrCl 4 instead of ZrO 2 or isopropanol. Indeed, during the decomposition of ZrCl 2 (O i Pr) 2 , the 31 P NMR spectrum shows evidence of a transient ZrCl 3 (O i Pr) species. The third (autocatalyzed) step can thus be conceived as the decomposition of ZrCl