Abstract. In this paper we consider boundary control problems associated to a semilinear elliptic equation defined in a curved domain Ω. The Dirichlet and Neumann cases are analyzed. To deal with the numerical analysis of these problems, the approximation of Ω by an appropriate domain Ω h (typically polygonal) is required. Here we do not consider the numerical approximation of the control problems. Instead, we formulate the corresponding infinite dimensional control problems in Ω h , and we study the influence of the replacement of Ω by Ω h on the solutions of the control problems. Our goal is to compare the optimal controls defined on Γ = ∂Ω with those defined on Γ h = ∂Ω h and to derive some error estimates. The use of a convenient parametrization of the boundary is needed for such estimates. 1. Introduction. In this paper we study boundary control problems defined on a curved domain Ω. We start with the Neumann problem (NP) and consider the Dirichlet problem (DP) afterward. To numerically solve these problems, usually it is convenient to approximate Ω by a polygonal domain Ω h , e.g., if finite elements are used for computations. Our goal is to analyze the effect of the domain change on the optimal controls. More precisely, two new optimal control problems (NP h ) and (DP h ) in Ω h are defined. The convergence of global or local solutions of problems (NP h ) and (DP h ) to the corresponding local or global solutions of (NP) and (DP), respectively, is investigated for the limit passage h → 0. The error estimates for the difference of optimal controls obtained for both problems in an appropriate norm are derived in a function of the parameter h. We restrict ourselves to the case of a convex domain Ω ⊂ R 2 approximated by a polygonal domain Ω h ; h is the maximal length of the edges of Ω h . A family of infinite dimensional control problems (NP h ) and (DP h ) defined in Ω h is considered, and the solutions of (NP h ) and (DP h ) are compared with the solutions of (NP) and (DP), respectively. In this way, the influence of small changes in the domain on the solutions of the control problems is analyzed.The numerical computation of the solution of (NP) and (DP) requires the discretization of the respective state equations, typically by using finite elements. If Ω is a polygonal domain, then it is covered by the union of the triangles of the mesh, and Γ