Let A be a 0-sectorial operator with a bounded H ∞ (Σ σ )-calculus for some σ ∈ (0, π), e.g. a Laplace type operator on L p (Ω), 1 < p < ∞, where Ω is a manifold or a graph. We show that A has a H α 2 (R + ) Hörmander functional calculus if and only if certain operator families derived from the resolvent (λ−A) −1 , the semigroup e −zA , the wave operators e itA or the imaginary powerswe denote the R-bound of this set. In a Hilbert space X, R[L 2 (J)]-boundedness reduces to the simple estimate J | N(t)x, y | 2 dt 1 2 ≤ C x y for all x, y ∈ H.Assume now that A is a 0-sectorial operator with an H ∞ (Σ σ ) calculus for some σ ∈ (0, π) on a Banach space isomorphic to a subspace of an L p (Ω) space with 1 ≤ p < ∞ (or more generally, let X have Pisier's property (α)). Then our main results, Theorems 6.1 and 6.4 show (among other statements), that the following conditions on the operator function above are essentially equivalent:1. A has an R-bounded H α 2 spectral calculus, i.e.