A series of uranium(VI)-acetylide complexes of the general formula U-vI(O)(C C-C6H4-R)[N(SiMe3)2]3, with variation of the para substituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, was prepared. These compounds were analyzed by C-13 NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, C-Ar, was shifted strongly downfield, with i5(C-13) values ranging from 392.1 to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. These extreme high-frequency '3C resonances are attributed to large negative paramagnetic (o"P') and relativistic spin-orbit (o-s) shielding contributions, associated with extensive U(Sf) and C(2s) orbital contributions to the U-C bonding in title complexes. The trend in the C-13 chemical shift of the terminal acetylide carbon is opposite that observed in the series of parent (aryl)acetylenes, due to shielding effects of the para substituent. The C-13 chemical shifts of the acetylide carbon instead correlate with DFT computed U-C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibility measurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes, suggesting a magnetic field-induced mixing of the singlet ground-state (sigma) of the U(VI) ion with low-lying (thermally inaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually large paramagnetic and SO contributions to the C-13 shifts). Thus, together with reported data, we demonstrate that the sensitive '3C NMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.