This paper considers the Dirichlet problem, where a is a scalar diffusion function. For a fixed f , we discuss under which conditions is a uniquely determined and when can a be stably recovered from the knowledge of u a . A first result is that whenever a ∈ H 1 (D), with 0 < λ ≤ a ≤ Λ on D, and f ∈ L ∞ (D) is strictly positive, thenMore generally, it is shown that the assumption a ∈ H 1 (D) can be weakened to a ∈ H s (D), for certain s < 1, at the expense of lowering the exponent 1/6 to a value that depends on s.