Abstract. We give a Dirichlet form approach for the construction and analysis of elliptic diffusions in Ω ⊂ R n with reflecting boundary condition. The problem is formulated in an L 2 -setting w.r.t. a reference measure µ on Ω having an integrable, dx-a.e. positive, density w.r.t. the Lebesgue measure. The symmetric Dirichlet forms (E ,a , D(E ,a )) we consider are the closure of the symmetric bilinear forms, where a is a symmetric, elliptic, n × n-matrix-valued measurable function on Ω. Assuming that Ω is an open, relatively compact set with boundary ∂Ω of Lebesgue measure zero and that is satisfying the Hamza condition, we can show that (E ,a , D(E ,a )) is a local, quasi-regular Dirichlet form. Hence, it has an associated self-adjoint generator (L ,a , D(L ,a )) and diffusion process M ,a (i.e, an associated strong Markov process with continuous sample paths). Furthermore, since 1 ∈ D(E ,a ) (due to the Neumann boundary condition) and E ,a (1, 1) = 0, we obtain a conservative process M ,a (i.e., M ,a has infinite life time). Additionally assuming that √ ∈ W 1,2 (Ω) ∩ C(Ω) or that is bounded, Ω is convex and { = 0} has codimension ≥ 2 we can show that the set { = 0} has E ,a -capacity zero. Therefore, in this case we even can construct an associated conservative diffusion process in { > 0}. This is essential for our application to continuous N -particle systems with singular interactions. Note that for the construction of the self-adjoint generator (L ,a , D(L ,a )) and the Markov process M ,a we do not need to assume any differentiability condition on and a. The following explicit representation of the generator we obtain for √ ∈ W 1,2 (Ω) and a ∈ W 1,∞ (Ω):∂i(aij ∂j) + ∂i(log ) aij ∂j.Note that the drift term can be very singular, because we allow to be zero on a set of Lebesgue measure zero. Our assumptions in this paper even allow a drift which is not integrable w.r.t. the Lebesgue measure.