We present a study of photoinduced interfacial electron transfer (ET) dynamics of SnO2 nanocrystalline thinfilms sensitized by polythiophene derivatives (regioregular poly(3-hexylthiophene) (P3HT) and regiorandompoly(3-undecyl-2,2‘-bithiophene) (P3UBT)). ET dynamics were measured by following the dynamics of injectedelectrons in SnO2 and polarons in the conjugated polymer using ultrafast mid-IR transient absorptionspectroscopy. The rate of electron transfer from P3HT and P3UBT to SnO2 films was determined to occur onsub-picosecond time scale (120 ± 20 fs). In P3HT/SnO2 composite, interchain charge transfer was found tocompete with and reduce the quantum efficiency of interfacial electron transfer at high polymer loading. Thisinterchain charge separation processes can be reduced in non-regioregular polymer or at low polymer loadinglevels.