A variety of zirconocene thioaldehyde complexes 1 have been prepared in high yield by heating (alkylthio)-methylzirconocenes 2 in the presence of PMe3. The formation of the thiobenzaldehyde complex (1b) from Cp2Zr(Me)SCH2Ph (2b) has been found to follow first-order kinetics independent of trimethylphosphine concentration, with ΔH‡ = 18.6 (0.1) kcal/mol and ΔS‡ = -20.6 (0.4) eu. A large primary kinetic isotope effect (kH / kD = 5.2 (0.2) at 80 oC) was observed for the transformation. A Hammet plot showed that phenyl substituents exert a modest influence on the reaction rate with p = +0.39 (0.02). These observations are more consistent with a concerted four-center cyclometalation process in which the transition state is polarized so that hydrogen moves as a proton than with a stepwise β-hydride elimination/reductive elimination sequence.