Isotopically labeled zirconocene methyl neopentyl complexes of the formula (CpRn)2Zr(CH3)(CH2C(CH3)2CD3) are obtained via reaction of LiCH2C(CH3)2CD3 with (CpRn)2Zr(CH3)(Cl). Addition of B(C6F5)3 to (CpRn)2Zr(CH3)(CH2C(CH3)2CD3) results in β-methyl elimination, forming the ion-paired species [(CpRn)2Zr(CH3)][CH3B(C6F5)3] along with isotopologs of isobutene. The relative amounts of d3- and d0-isobutene afford the isotope effect for β-methyl elimination. For Cp2Zr(CH3)(CH2C(CH3)2CD3) (Cp = (η5-C5H5)), a kinetic deuterium isotope effect of 1.40(2) has been measured at 23 °C. Comparable deuterium kinetic isotope effects have been observed for four other zirconocene methyl neopentyl compounds: [Cp*(C5Me4H)Zr] (Cp* = (η5-C5Me5)), [Cp*2Zr], [rac-(EBI)Zr] (EBI = ethylenebis(indenyl)), and [(THP)Zr] (THP = 1,2-(SiMe2)2(η5-3,5-C5H(CHMe2)2)(η5-C5H3)). The direction and magnitude of these effects are consistent with γ-agostic assistance in the transition state for β-methyl elimination, the microscopic reverse of α-agostic assistance in the transition state for olefin insertion into the Zr−methyl bond.