The synchrotron photoionization method was applied to measure L-subshell Coster-Kronig yields. This method is based on the capability of tuning the energy of the synchrotron photons producing a selective subshell ionization. Two foil samples of Yb and Ta were irradiated and their characteristic spectra were recorded. Data were analyzed using a new formalism (based on a matrix representation) for expressing X-ray fluorescence intensities involving Coster-Kronig transitions. The results obtained in this work are f 12 = 0.249 ± 0.021, f 13 = 0.408 ± 0.055 and f 23 = 0.186 ± 0.040 for Yb, and f 12 = 0.168 ± 0.039, f 13 = 0.322 ± 0.072 and f 23 = 0161 ± 0.053 for Ta. These data are very reliable and represent a valuable information for spectroscopists, considering the lack of data for L-shell parameters.