Abstract. Few programming languages have a mathematically rigorous definition or metatheory—in part because they are perceived as too large and complex to work with. This paper demonstrates the feasibility of such undertakings: we formalize a substantial portion of the semantics of Objective Caml’s core language (which had not previously been given a formal semantics), and we develop a mechanized type soundness proof in HOL. We also develop an executable version of the operational semantics, verify that it coincides with our semantic definition, and use it to test conformance between the semantics and the OCaml implementation. We intend our semantics to be a suitable substrate for the verification of OCaml programs. 1 Mechanizing Metatheory Researchers in programming languages and program verification routinely develop their ideas in the context of core calculi and idealized models. The advantage of the core calculus approach comes from the efficacy of pencil-and-paper mathematics, both for specification and proof; however, these techniques do not scale well. Usable programming