The electronic properties of InxGa(1-x)As(1-y)Ny /GaAs single quantum wells have been investigated by photoluminescence and photoreflectance spectroscopy as a function of temperature. The introduction of nitrogen leads to a sizable slow down in the redshift of the ground state recombination energy with temperature. We explain the observed effects in terms of an anticrossing between states of the conduction band (CB) edge and a N-induced localized level resonant with the CB. The extent of this anticrossing, described by the matrix element V(MN) , is derived from the temperature dependence of the exciton recombination energy in a wide compositional range. The measured functional dependence of V(MN) on nitrogen concentration is compared with results reported in the literature.