We investigated influence of measurement temperature and microwave power on the isotopic shift in fine structure term of spin Hamiltonian of the NL19 ESR spectra, which was attributed to an iron–vacancy pair defect in silicon. Obtained results can be described by the model implying existence of discrete local electron-vibration states. Microwave power dependencies of the signal and the isotopic shift at temperatures below 8K suggest active role of local vibrations of the iron atoms in spin-system relaxation.