Comprehensive Raman studies of lattice vibrations in oxide superconductors BaPb1-xBixO3 are presented. In order to explain consistently the Raman and infrared data a dimerization model is proposed for the appearance of the semiconducting state in this system. The transfer-integral, on-site Coulomb repulsion and electron-molecular-vibration coupling constant are presented as a function of Bi concentration. The calculated activation energy is in good agreement with the experimental data.