We propose a double exchange model to describe the RuO 2 planes of RuSr 2 (Eu,Gd)Cu 2 O 8 . The Ru +5 ions are described by localized spins, and additional electrons provided by the superconducting CuO 2 planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru +5 spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.