Let $X$ be a locally compact space, $m$ a Radon measure on $X$, $\h$ a regular 
Dirichlet form in $L_2(X,m)$. For a Radon measure $\mu$ we interpret $\h$
as a regular Dirichlet form $\tau$ in $L_2(m+\mu)$. We show that $\mu$ decomposes as
$\mu_r+\mu_s$ where $\mu_r$ is coupled to $\h$ and $\mu_s$ decouples from $\h$.
Additionally to this `space perturbation', a second perturbation is introduced
by a measure $\nu$ describing absorption.

The main object of the paper is to apply this setting to a study of the
Wentzell boundary condition
\[
   -\alpha Au + n\cdot a\nabla{}u + \gamma u = 0 \quad \text{on}\
\partial\Omega 
\]
for an elliptic operator $A=-\nabla{}\<\cdot(a\nabla{})$, where
$\Omega\subseteq\R^d$ is open, $n$ the outward
normal, and $\alpha$, $\gamma$ are suitable functions. 
It turns out that the previous setting can be applied with 
$\mu=\alpha dS$,\, $\nu=\gamma dS$, under suitable conditions.
Besides the description of the $d$-dimensional case we give a more
detailed analysis of the one-dimensional case.

As a further topic in the general setting we study the qestion whether mass
conservation carries over from the unperturbed form $\h$ to the space perturbed
form $\tau$. In an appendix we extend a known closability criterion from the
minimal to the maximal form.