Electron-doping of the semimetal (CeOFeAs) by either fluorine (max T c ∼ 43 K) or cobalt (max Tc ∼ 11 K) leads to superconductivity. Here we show the effect of transition metal (Co) substitution at the iron site on the superconducting properties of CeO0.9F 0.1FeAs (Tc ∼ 38 K) to understand the interplay of charge carriers in both the rare earth-oxygen and Fe-As layers. Simultaneous doping of equivalent number of charge carriers in both layers leads to a T c of 9.8 K which is lower than the Tc obtained when either the conducting layer (FeAs) or charge reservoir layer (CeO) is individually doped. This suggests a clear interplay between the two layers to control the superconductivity. Resistivity upturn and negative magnetoresistance are observed with Co doping that is interpreted in the gamut of Kondo effect. Hall coefficient and thermoelectric power indicate increased carrier concentration with cobalt doping in CeO0.9F0.1FeAs. The rf penetration depth both for CeO0.9F0.1Fe0.95Co 0.05As and CeO0.9F0.1FeAs show an exponential temperature dependence with gap values of ∼1.6 and 1.9 meV respectively. © 2010 Elsevier B.V. All rights reserved.