Abstract. Let X be a smooth complex projective curve of genus g ≥ 2. A pair on X is formed by a vector bundle E → X and a global non-zero section φ ∈ H 0 (E). There is a concept of stability for pairs depending on a real parameter τ , giving rise to moduli spaces M τ (r, Λ) of τ -stable pairs of rank r and fixed determinant Λ. In this paper we prove that the moduli spaces M τ (r, Λ) are in many cases rational.Dedicated to the 60th Anniversary of Themistocles M. Rassias