  We consider the inverse boundary value problem for the steady state
convection diffusion equation. We prove that a velocity field $V$, is uniquely
determined by the Dirichlet-to-Neumann map, when $V \in C^{0,\gamma} (\Omega)$,
$2/3< \gamma \leq 1$, i.e. when $V$ is a H\"older continuous vector field with
$2/3< \gamma \leq 1$.
