3p and 3d core-level widths have been determined experimentally by the help of internal-conversion electron spectroscopy in metallic $^{99\mathit{m}}\mathrm{Tc}$. The Coster-Kronig broadening of an internal-conversion line has been identified in the case of the ${\mathit{M}}_{4}$ transition. The existence of chemical effects on the 3${\mathit{d}}_{3/2}$ core-hole lifetime is shown for ${\mathrm{NH}}_{4}$${\mathrm{TcO}}_{4}$. From the measured energies of the conversion peaks and from the core-level binding energies obtained earlier by x-ray photoelectron spectroscopy, the value of the E3 transition energy has been determined to be 2173.8\ifmmode\pm\else\textpm\fi{}0.4 eV. Application possibilities of the results in resolution calibration of electron spectrometers and for x-ray width determination are briefly discussed.