The gamma ray and conversion electron spectra of the 22.5 keV M1 + E2 nuclear transition depopulating the first excited level 5/2(-) in Sm-149 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 vertical bar delta(E2/M1)vertical bar = 0.0784 +/- 0.0009 and the nuclear structure parameter lambda = -2 +/- 10 were derived. The vertical bar delta(E2/M1)vertical bar and lambda parameters were also obtained for all available experimental data and their mutual combination.