Nonlinear optical measurements were performed to elucidate the influence of magnetic ions on the behavior of charge carriers in magnetic semiconductors-Pb1-xYbxX (X = S, Se, Te at x = 1-3%). It was shown that nonlinear optical methods could be used as sensitive tools for investigations of electron-phonon anharmonicity near low-temperature semiconductor-insulator phase transitions. There exists a difference between surface and bulk-like contributions to the nonlinear optical effects. It was shown that only low-temperature Two Photon Absorption (TPA) oscillator may be related to the number of the electron-phonon anharmonic modes responsible for the observed phase transformation. The explanation of the anomalous temperature dependences is given in accordance with dipole momentum's behaviors determined by low-temperature spin-spin interactions and by electron-phonon anharmonic interactions. We have discovered that low-temperature dependence of specific heat of Pb1-xRxTe (R = Yb, Pr with x = 3% and 1.6%, respectively) exhibits a non-magnetic order caused by large electron-phonon contributions and structural disorder effects. (c) 2006 Elsevier Ltd. All rights reserved.