Using a xenon arc lamp source with a spectral distribution extending from 10 000 to 5000 Å, we have found it possible to generate a detectable nonlinear sum spectrum in ADP over the range from 5000 to 2700 Å. This sum signal distribution has a spectrometer‐limited resolution of 20 Å which is sufficiently good to resolve line structure and to permit a comparison of this structure with the xenon lamp line spectrum. It is found that the greater part of the sum signal consists of a structureless background which is attributed to nonlinear mixing involving the xenon blackbody continuuum spectrum. A computer program was used to calculate the expected sum signal distribution assuming that the nonlinear polarization constant is independent of frequency. The calculated spectrum agrees qualitatively with the sum signal spectrum obtained experimentally.