Abstract. Let R be a commutative, indecomposable ring with identity and (P, ≤) a partially ordered set. Let F I(P ) denote the finitary incidence algebra of (P, ≤) over R. We will show that, in most cases, local automorphisms of F I(P ) are actually R-algebra automorphisms. In fact, the existence of local automorphisms which fail to be R-algebra automorphisms will depend on the chosen model of set theory and will require the existence of measurable cardinals. We will discuss local automorphisms of cartesian products as a special case in preparation of the general result.

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