The authors have determined the composition profile within individual Si_{1 - x}Ge_x nanoscale islands on Si(001). Samples have been grown by means of liquid phase epitaxy in the Stranski-Krastanov mode. By applying electron energy loss spectroscopy, the intensities of Si K and Ge L edges have been measured to determine the relative atomic concentration of germanium. The quantification of the composition suggests a profile comprising of two regions with different linear concentration gradients.