Let be a von Neumann algebra, let σ be a strongly continuous representation of the locally compact abelian group G as ^*-automorphisms of . Let M(σ) be the Banach algebra of bounded linear operators on generated by ∝ σ_t dμ(t) (μ ε M(G)). Then it is shown that M(σ) is semisimple whenever either (i) has a σ-invariant faithful, normal, semifinite, weight (ii) σ is an inner representation or (iii) G is discrete and each σ_t is inner. It is shown that the Banach algebra L(σ) generated by ∝ ƒ(t)σ_t dt (ƒ ε L¹(G)) is semisimple if a is an integrable representation. Furthermore, if σ is an inner representation with compact spectrum, it is shown that L(σ) is embedded in a commutative, semisimple, regular Banach algebra with isometric involution that is generated by projections. This algebra is contained in the ultraweakly continuous linear operators on . Also the spectral subspaces of σ are given in terms of projections.