EN
We investigate the spin resonance of electrons in one-sided modulation doped Si_{1-x}Ge_x (x=0-10%)) quantum wells defined by Si_{0.75}Ge_{0.25} barriers. In such structures, the Bychkov-Rashba effect induces an effective magnetic field in the quantum well layer which causes anisotropy of both the g-factor and the spin coherence time. Evaluation of the Rashba coefficient as a function of x yields a monotonic increase. For x=5% the shift in the resonance field exceeds the ESR linewidth already, demonstrating the possibility to use this effect for g-factor tuning to select individual spins in an ensemble.