Abstract. We develop duality-based a posteriori error estimates for functional outputs of solutions of free-boundary problems via shape-linearization principles. To derive an appropriate dual (linearized adjoint) problem, we linearize the domain dependence of the very weak form and goal functional of interest using techniques from shape calculus. We show for a Bernoulli-type free-boundary problem that the dual problem corresponds to a Poisson problem with a Robin-type boundary condition involving the curvature. Moreover, we derive a generalization of the dual problem for nonsmooth free boundaries which includes a natural extension of the curvature term. The effectivity of the dualbased error estimate and its usefulness in goal-oriented adaptive mesh refinement is demonstrated by numerical experiments.Key words. goal-oriented error estimation, a posteriori error estimation, Bernoulli freeboundary problem, shape derivative, shape differential calculus, linearized adjoint, adaptive mesh refinement AMS subject classifications. 35R35, 49M29, 54C56, 58C20, 65N15, 65N50 DOI. 10.1137/080741239 1. Introduction. This is the shape-linearization part of our work on goaloriented error estimation and adaptivity for free-boundary problems; see also [42]. We consider duality-based a posteriori error estimates for functional outputs that include the dependence on both the error in the approximate solution and the error in the domain approximation.In [42], we explained that free-boundary problems elude the standard goaloriented error estimation framework because their typical variational form is noncanonical. In pursuit of a canonical form, we introduced the domain-map linearization approach at a reference domain which in essence reformulates the free-boundary problem to a fixed reference domain. Accordingly, the dual (linearized adjoint) problem is obtained by linearizing the transformed problem with respect to the domain map. This approach is straightforward. However, the dual problem contains nonstandard and nonlocal interior and boundary terms, which is inconvenient from an implementation point of view. Moreover, there is some arbitrariness in the dual problem due to the heuristic extension of boundary perturbations into the domain. A similar arbitrariness appears in shape optimization in the so-called material derivative approach [23,36]. An elegant alternative in shape optimization is the shape derivative whose variational formulation consists only of standard interior and boundary terms.