The evolution of the photoinduced birefringence in thin films of narrow polymer fractions is studied and compared with the behavior of the non-fractionated polymer. The Delta n(ind) value decreases by increasing the degree of polymerization (DP) within the oligomeric range but becomes independent of molecular weight starting from a DP of approximate to 70. Thermal pretreatment of the films results in higher photoinduced birefringence. The films show good stability of the photorecording.