Electric permittivity and density were measured in a nitrobenzene and octane mixture in the vicinity of the upper critical consolute point. Measurements were conducted in the one-phase region, at the critical concentration. The possibility of stirring in the course of measurements allowed us to check if the density and concentration gradients had any influence on the obtained results. No signs of the presence of the gradients mentioned above were found. Using the data obtained in the reported measurements, different methods of the fitting of the equation describing the permittivity anomaly were tested. The calculation of a reliable value of the critical amplitude, used to estimate the critical temperature shift under the influence of the electric field, was of particular interest. The derivative (partial derivative T-c/partial derivative E-2) was found to be (-3.9 +/- 0.3) X 10(-16) K m(2) V-2.