Abstract. Let M be the moduli space of rank 2 stable torsion free sheaves with Chern classes c i on a smooth 3-fold X. When X is toric with torus T , we describe the T -fixed locus of the moduli space. Connected components of M T with constant reflexive hulls are isomorphic to products of P 1 . We mainly consider such connected components, which typically arise for any c 1 , "low values" of c 2 , and arbitrary c 3 .In the classical part of the paper, we introduce a new type of combinatorics called double box configurations, which can be used to compute the generating function Z(q) of topological Euler characteristics of M (summing over all c 3 ). The combinatorics is solved using the double dimer model in a companion paper. This leads to explicit formulae for Z(q) involving the MacMahon function.In the virtual part of the paper, we define Donaldson-Thomas type invariants of toric Calabi-Yau 3-folds by virtual localization. The contribution to the invariant of an individual connected component of the T -fixed locus is in general not equal to its signed Euler characteristic due to T -fixed obstructions. Nevertheless, the generating function of all invariants is given by Z(q) up to signs.