New method is suggested and applied for an estimation of the delocalization length of charge carriers in photoconductive polymers based on the magnetic field effect (MFE) on the photoconductivity (PC). The method was applied to photoconductive polymers PTS, PPV, PPPV, DMOP-PPV and has permitted to estimate delocalization lengths as large as 60 Å. It makes questionable the usage of Onsager's model of charge separation for conjugated polymers (CP).