For conduction electrons moving in quantum wells, a transverse electric field appears as an effective magnetic field causing spin relaxation via the Dyakonov-Perel mechanism. For undoped samples, the strength of the effective magnetic field for a particular electric field and temperature is determined by the Rashba coefficient. By combining spin grating and relaxation measurements, we have measured the Rashba coefficient for GaAs/AlGaAs quantum wells. There is good agreement with k.p theory at low temperature. We observe a deviation from the theoretical value at higher temperatures which could be a result of higher order terms in the Rashba spin-orbit interaction not normally considered.