We prove that for any n >= 2, the abstract commensurator group of the Baumslag-Solitar group BS(1, n) is isomorphic to the subgroup {((0) (1)(p) (q)) vertical bar q is an element of Q, p is an element of Q*} of GL(2)(Q). We also prove that for any finitely generated group G with the unique root property the natural homomorphisms Aut(G) -> Comm(G) -> QI(G) are embeddings.