CuGeO3 is a good realization of a spin-Peierls system. Using neutron scattering, we have investigated the transition from the dimerized spin-Peierls phase to an incommensurately modulated high-field phase. The incommensurate period has been measured for fields up to 14.5 T and is found to be in excellent accord with the theoretical predictions. The intensities of the satellite reflections provide evidence for the existence of a magnetic contribution with the same modulation period as the structural.