Quantum mechanical studies of polaron formation in oligo[methyl(phenyl)silane] (OMPSi) was performed. The conformations of the neutral, positively and negatively charged OMPSi were optimized by means of B3LYP method. For the evaluation of the substituent effects, the conformation of the unsubstituted oligosilane chain was also calculated. The negative polaron conformation shows a significant stretching of the chain, whereas the positive polaron conformation is shrunk. This fact is associated with changes of electron density distribution. There is also a strong influence of the pi-conjugated substituent on the conformation of the negative polaron. On the other hand, the influence of the substituents on the conformation of the positive polaron is small. The absorption spectra of the defect-free and polaron oligomers are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.