The isotropic average, sigma(Hg)(iso), and the anisotropy, Deltasigma(Hg), of the Hg-199 shielding tensor in methylmercury halides, CH3HgX (X = Cl, Br, I), were determined by studying the Hg-199 NMR of these molecules dissolved in liquid crystals. Furthermore, density functional calculations were performed using the zeroth order regular approximation, inluding also dimethylmercury. Detailed comparison of the experimental and calculated results is problematic because of the sensitivity of the Hg-199 shielding to environmental effects. It is, however, clear that calculations with spin-orbit interaction taken into account lead to the sigma(Hg)(iso) and Deltasigma(Hg) values that predict the same trend as the experiments; the chemical shift relative to dimethylmercury, sigma(Hg)(iso)((CH3)(2)Hg) -sigma(Hg)(iso)(CH3HgX), increases while the shielding anisotropy decreases along the series (CH3)(2)Hg to CH3HgI.