The compounds we want to study belong to the well-known tetragonal RM2X2 family (R: rare earth, M: d element, X: Ge, Si). In EuCu2(SixGe1-x)2, the Eu valence changes, as a function of the Si concentration x, from close to 2+ (magnetic, 4f7) for x = 0 to strongly intermediate, nonmagnetic above xc = 0.65. In the vicinity of xc, the system exhibits Kondo effect and heavy-fermion properties, which are quite unusual for Eu. We propose to probe the sd-f hybridization in these materials by studying its effects on the crystal-field scheme of Er impurities, which can sense the delocalization of 4f electrons. Another goal is to check the absence of ionic Eu3+ excitations in Ge-rich compounds despite the sizable deviation from the 2+ valence. The measurements can be done on MARI with sufficiently thin samples and using a low enough Er concentration so as not to affect the properties of the host.