Abstract. Let G be an algebraic group over a field F . As defined by Serre, a cohomological invariant of G of degree n with values in Q/Z(j) is a functorial in K collection of maps of sets H 1 (K, G) −→ H n K, Q/Z(j) for all field extensions K/F . We study the group of degree 3 invariants of an algebraic torus with values in Q/Z(2). In particular, we compute the group H 3 nr F (S), Q/Z(2) of unramified cohomology of an algebraic torus S.
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