A combined theoretical and experimental investigation into the structure and mechanism of the classical Vandenberg catalyst for the isoselective polymerization of epoxides has led to a consistent mechanistic proposal. The most likely reaction pathway was based on a bis(μ-oxo)di(aluminum) (BOD) resting state that proceeded through a mono(μoxo)di(aluminum) (MOD) transition state. The isoselectivity of the Vandenberg catalyst was derived from the rigidity of the BOD structure and its bonding to the ultimate and penultimate oxygen heteroatoms along the polyether backbone. The energetic driving force for isoselectivity was the loss of an energetically favorable secondary Al−O interaction during enchainment of oppositely configured epoxides, providing a ca. 2 kcal/mol driving force for the emergent isoselectivity. Experimental spectroscopic and kinetic evidence based on model BOD and MOD complexes support the new mechanistic framework developed using density functional theory calculations. A purposefully synthesized BOD analogue of the proposed Vandenberg structure produced a characteristically isotactically enriched poly(allyl glycidyl ether) as produced by the classical Vandenberg catalyst. In situ 1 H NMR spectroscopy of a Vandenberg-catalyzed polymerization of allyl glycidyl ether revealed the activation enthalpy (ΔH ‡ = 21 kcal/mol) and energetics of epoxide−aluminum coordination (ΔH = −4.0 ± 1.0 kcal/mol, ΔS = −0.018 ± 0.004 kcal/(K mol)) by observation of the shifting acetylacetonate signal located on the active site of the Vandenberg catalyst in the 1 H NMR spectra of polymerization.