  We extend the classical Mercer theorem to reproducing kernel Hilbert spaces
whose elements are functions from a measurable space $X$into $\mathbb C^n$.
Given a finite measure $\mu$ on $X$, we represent the reproducing kernel $K$ as
convergent series in terms of the eigenfunctions of a suitable compact operator
depending on $K$ and $\mu$. Our result holds under the mild assumption that $K$
is measurable and the associated Hilbert space is separable. Furthermore, we
show that $X$ has a natural second countable topology with respect to which the
eigenfunctions are continuous and the series representing $K$ uniformly
converges to $K$ on any compact subsets of $X\times X$, provided that the
support of $\mu$ is $X$.
