  We consider a reaction--diffusion equation perturbed by noise (not
necessarily white). We prove an integral inequality for the invariant measure
$\nu$ of a stochastic reaction--diffusion equation. Then we discuss some
consequences as an integration by parts formula which extends to $\nu$ a basic
identity of the Malliavin Calculus. Finally, we prove the existence of a
surface measure for a ball and a half-space of $H$.
