We measured the unclamped electro-optic coefficients $r_{13}$ and $r_{33}$ of near-stoichiometric and congruent lithium tantalate $\left(\mathrm{Li}\mathrm{Ta}{\mathrm{O}}_3\right)$ from $\lambda =633\mathrm{nm}$ down to the absorption edge at $\sim 275\mathrm{nm}$ . Electro-optical coefficients up to $r_{33}=52\pm 1\mathrm{pm}∕\mathrm{V}$ have been determined at $\lambda =275\mathrm{nm}$ . The reduced half-wave voltage at $275\mathrm{nm}$ is $v_{\pi }=300\pm 10\mathrm{V}$ for $r_{33}$ and $v_{\pi }=560\pm 20\mathrm{V}$ for the configuration employing $r_c=r_{33}-{(n_o∕n_e)}^3{r}_{13}$ . We propose a two-oscillator polarization potential model that well describes our data in the UV. In the investigated wavelength range the values for stoichiometric and congruent $\mathrm{Li}\mathrm{Ta}{\mathrm{O}}_3$ differ by less than 3%, which is below the accuracy of our measurements. In addition, the refractive indices in the UV were measured, and more precise Sellmeier parameters for the UV and visible were determined based on the previous and new refractive-index data.