GaAs/InGaAs/GaAs quantum well structures grown on GaAs (001) by molecular beam epitaxy (MBE) at substrate temperatures between 450 degrees C and 540 degrees C are investigated using X-ray diffraction (XRD) techniques. As the substrate temperature increases, the diffraction peak angles of InGaAs shift to the lower In composition side, and the integrated diffraction spectrum of the InGaAs peaks weighted by composition x decreases. By assuming a constant segregation rate, we estimate the In desorption and segregation rates, and determine the In distribution profiles of the quantum well structures.