A computer method is described for making after‐the‐fact adjustments for incorrect zero energy intercept and instrumental gain to experimental gamma ray pulse height spectra from NaI (Tl) scintillation spectrometers. The method makes use of the 0.511 MeV annihilation peak and 1.46 MeV40K photopeak as internal energy reference standards, both of which peaks are available to NaI (Tl) scintillation detectors. The technique has been tested experimentally and appears to be of practical use for routine energy standardization of long counting time gamma ray spectra between 0.1 and 2.0 MeV.