Abstract. Suppose that F (x) ∈ Z [[x]] is a Mahler function and that 1/b is in the radius of convergence of F (x) for an integer b ≥ 2. In this paper, we consider the approximation of F (1/b) by algebraic numbers. In particular, we prove that F (1/b) cannot be a Liouville number. If, in addition, F (x) is regular, we show that F (1/b) is either rational or transcendental, and in the latter case that F (1/b) is an S-number or a T -number in Mahler's classification of real numbers.