The effect of ytterbium substitution at the lanthanum site on the superconducting properties of La1-xYbxO 0·8F0·2FeAs (x = 0·10, 0·20 and 0·30) oxypnictides has been investigated. Powder X-ray diffraction studies show the presence of Yb2O3 and LaOF as secondary phases. The superconducting transition temperature (Tc) of 31·3 (± 0·05) K has been observed in x = 0·1 composition which is the maximum Tc so far in the La(O/F)FeAs superconductor family at ambient pressure. Further increase in x leads to suppression and broadening of superconducting transition. The resistive transition curves under different magnetic fields were investigated, leading to determination of upper critical field Hc2 (T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 46 T corresponding to coherence length ∼27 Å. © Indian Academy of Sciences.