Compositional dependences of the optoelectronic properties of sol–gel derived CdO–ZnO composite films with volume ratio of Cd:Zn ranging from 1:0 to 0:1 (with a step of 1/4) have been studied. After heat treatments in air the prepared thin films were investigated by studying their structural, morphological, d.c. electrical and optical properties. X-ray diffraction (XRD) results suggest that the samples are polycrystalline and the crystallinity of them increased with Cd ratio. The average grain size is in the range of 20–53 nm. As composition and structure changed due to the Cd volume ratio, the order of the carrier concentration was varied from 10 16 to 10 20 cm −3 with Cd ratio and the mobility increased from less than 2 to 45 cm 2 V −1 s −1. It was found that the transmittance and the band gap decreased as Cd ratio increased. The optical constants of the film were studied and the dispersion of the refractive index was discussed in terms of the Wemple–DiDomenico single oscillator model. The real and imaginary parts of the dielectric constant of the films were determined too. The volume energy loss (VELF) increases more than the surface energy loss (SELF) at their particular peaks. The third-order nonlinear polarizability parameter is higher for CdO–ZnO thin films with higher concentration of cadmium oxide. Display omitted • Physical properties of the sol–gel derived CdO–ZnO nanostructured thin films prepared at different Cd content have been investigated. • The carrier concentration and carrier mobility of the samples were increased with Cd ratio whereas the resistivity was decreased. • The direct optical band gap shifted to the lower energy. • The refractive index and extinction coefficient increase with the increasing of Cd content. • The single-oscillator energy values are correlated with the optical band gap values by an empirical relation: E o ≈ 2 E g . • With the increase in Cd content nonlinear optical susceptibility χ (3) increases. Compositional dependence of the optoelectronic properties of sol–gel derived CdO–ZnO composite films with volume ratio of Cd:Zn ranging from 1:0 to 0:1 (with a step of 1/4) has been studied. After heat treatments in air the prepared thin films were investigated by studying their structural, morphological, d.c. electrical and optical properties. X-ray diffraction (XRD) results suggest that the samples are polycrystalline and the crystallinity of them increased with Cd ratio. The average grain size is in the range of 20–34 nm. As composition and structure changed due to the Cd volume ratio, the order of the carrier concentration was varied from 10 16 to 10 20 cm −3 with Cd ratio and the mobility increased from less than 2 to 45 cm 2 V −1 s −1. It was found that the transmittance and the band gap decreased as Cd ratio increased. The optical constants of the film were studied and the dispersion of the refractive index was discussed in terms of the Wemple–DiDomenico single oscillator model. The real and imaginary parts of the dielectric constant of the films were also determined. The volume energy loss (VELF) increases more than the surface energy loss (SELF) at their particular peaks. The third-order nonlinear polarizability parameter is higher for CdO–ZnO thin films with higher concentration of cadmium oxide.