A method of calibrating a lead-collimated, sodium iodide scintillation spectrometer is reported. Nine radioisotopes with gamma-ray energies in the range from 0.046 to 1.114 MeV were used. Details of the methods of data acquisition and reduction are presented. Methods are described for using response functions from the nine radioisotopes to generate response matrices for the energy range from 20 to 1200 keV. Techniques for unfolding experimental data using response matrices are compared, and a detailed error analysis is presented. For the routine analysis of experimental data using direct matrix inversion, a 25 × 25 response matrix with unequally spaced energy intervals over the range 20 to 1200 keV was found to be near optimum.