An analysis procedure for real time spectroscopic ellipsometry data is described that allows one to characterize compositional gradients in thin films prepared by continuously varying the deposition parameters. This approach provides the time evolution of (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the near‐surface dielectric function and composition of the film with a depth resolution as high as 7 A˚. We apply the analysis to obtain the depth profile of the relative void volume fraction for an amorphous silicon‐carbon alloy film prepared by plasma‐enhanced chemical vapor deposition. ©1995 American Institute of Physics.