Abstract. A conventional linear model for functional data involves expressing a response variable Y in terms of the explanatory function X(t), via the model: Y = a + I b(t) X(t) dt + error, where a is a scalar, b is an unknown function and I = [0, α] is a compact interval. However, in some problems the support of b or X, I 1 say, is a proper and unknown subset of I, and is a quantity of particular practical interest. In this paper, motivated by a real-data example involving particulate emissions, we develop methods for estimating I 1 . We give particular emphasis to the case I 1 = [0, θ], where θ ∈ (0, α], and suggest two methods for estimating a, b and θ jointly; we introduce techniques for selecting tuning parameters; and we explore properties of our methodology using both simulation and the real-data example mentioned above. Additionally, we derive theoretical properties of the methodology, and discuss implications of the theory. Our theoretical arguments give particular emphasis to the problem of identifiability.