The gamma ray and conversion electron spectra of the 22.5keV M1 + E2 nuclear transition depopulating the first excited level 5/2− in 149Sm were investigated in detail. A substantially improved value of 22500.2 ± 0.8 eV for the transition energy was determined that opens a possibility to establish an electron energy standard for the keV region based on the conversion electron lines of this transition. The E2 admixture parameter |δ(E2/M1)| = 0.0784 ± 0.0009 and the nuclear structure parameter λ = − 2 ± 10 were derived. The |δ(E2/M1)| and λ parameters were also obtained for all available experimental data and their mutual combination.