Isotope dilution gamma spectrometry (IDGS) for determination of the Pu concentration, using high  resolution gamma-ray spectrometry (HRGS) in the 40–150 keV energy range, is developed and described. The methodology involves purification of Pu by an anion exchange procedure, followed by the determination of Pu isotopic composition using HRGS. For isotope dilution, a precalibrated power reactor grade Pu (∼ 70 at.% ²³⁹Pu) solution from an Indian PHWR was used as a spike for research reactor grade Pu (∼ 95 at.% 239Pu) samples and vice versa. Changes in ²⁴⁰Pu/²³⁹Pu (45.24 keV/51.62 keV) and ²⁴¹Pu/²³⁹Pu (148.57 keV/129.29 keV) activity ratios in  the spiked samples were determined by gamma spectrometry. Using the atom ratios calculated from the measured activity ratios, the concentration of Pu in the sample was calculated using two different efficiency calibration methods. The isotopic composition and concentration of Pu samples both from power reactor and research reactor grade agreed well with the values obtained by isotope dilution-thermal ionization mass spectrometry. The present method also shows the superiority of using the ²⁴⁰Pu/²³⁹Pu atom ratio against the ²⁴¹Pu/²³⁹Pu atom ratio in IDGS with respect to the accuracy  and precision.