We consider the Kolmogorov operator associated with a reaction-diffusion equation having polynomially growing reaction coefficient and perturbed by a noise of multiplicative type, in the Banach space E of continuous functions. By analyzing the smoothing properties of the associated transition semigroup, we prove a modification of the classical identité du carré di champs that applies to the present non-Hilbertian setting. As an application of this identity, we construct the Sobolev space W 1,2 (E; µ), where µ is an invariant measure for the system, and we prove the validity of the Poincaré inequality and of the spectral gap. * Key words and phrases: Stochastic reaction-diffusion equations, Kolmogorov operators, Poincaré inequality, spectral gap, Sobolev spaces in infinite dimensional spaces . † Partially supported by the NSF grant DMS0907295 "Asymptotic Problems for SPDE's".and this allowed us to prove that the series in (1.5) is convergent, for any ϕ ∈ D(K). To this purpose, we would like to mention the fact that our duality argument does work because we