A promising approach to improve the processability of semiconducting polymers is their incorporation into host matrices formed by conventional polymers such as polyethylene (PE) or polystyrene (PS). We:have characterized the Linear optical properties of several guest-host systems by absorption and luminescence measurements and probed the excited states by photoinduced absorption measurements (PIA) and light induced electron spin resonance (LESR). The:interesting photophysics of conjugated polymer/fullerene donor-acceptor system,showing an ultrafast photoinduced electron transfer and a metastable charge separation in the pristine state, has been investigated in such composites where the photoactive components are further embedded into a conventional polymer matrix. The composition of the blend and the relative concentration,of the components are found to strongly influence the photoinduced interaction between the conjugated polymer and the fullerenes (C-60 and functionalized fullerenes). Photoinduced electron transfer between a soluble polyphenylenevinylene (PPV) derivative and C-60 is observed in a system with PS as host when the concentration of the both electroactive; components is 33%. In, diluted-composites with PS as; matrix we find strong luminescence quenching of the:PPV derivative but no electron transfer upon adding C-60. Dominant photoexcitations in these PS systems are tripler states as follows from intensity dependencies and lifetime measurements. In diluted systems with PE as matrix neither charge transfer nor luminescence quenching has been observed.