The uniquely well-resolved Tc-99 NMR spectrum of the pertechnetate ion in liquid water poses a stringent test of the accuracy of ab initio calculations. The displacement of the Tc-99 chemical shift as a function of temperature has been measured over the range 10-45 degreesC for the three isotopomers Tc(O-16)(4)(-), Tc(O-16)(3)(O-18)(-), and Tc(O-16)(3)(O-17)(-) at natural oxygen isotope abundance levels, and in addition the temperature dependence of the Tc-O scalar coupling was determined for the Tc(O-16)(3)(O-17)(-) isotopomer. Values for these parameters were computed using relativistic spin-orbit density functional theory with an unsolvated ion approximation and with treatments of the solvated ion based on the COnductor-like Screening MOdel (COSMO) approach. The temperature and isotope dependence of Tc-99 NMR parameters inferred by these methods were in good quantitative agreement with experimental observations. The change in the Tc-O bond length associated with the changes in temperatures considered here was determined to be of the order of 10(-4) Angstrom. Vibrational energies and Tc-O bond lengths derived from these models also compare favorably with previous experimental studies.