In this work, comprehensive measurements of p rho T (pressure-density temperature) properties of cis-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz(Z)) were performed. Although this novel fluid has been proposed for various applications, including its use as a possible working fluid for organic Rankine cycle systems, reliable experimental data on its thermodynamic properties are scarcely available. In this work, the isochoric method was employed to obtain the properties of this fluid along 26 isochores for densities between 88 and 1295 kg.m(-3), and 344 p rho T data points were obtained in the temperature range from 323 K (49.85 degrees C) to 503 K (229.85 degrees C) and at pressures up to 10 MPa. The data covered single-phase liquid and vapor regions, two-phase region, and supercritical region. The data in the critical region were correlated with a simple polynomial expression, which was used to determine the critical parameters analytically. The vapor pressures and the densities of the saturated liquid and vapor were obtained from an analysis of the single- and two-phase data. The experimental uncertainties in the measurements of temperatures, pressures, densities below 100 kg.m(-3), and higher densities were estimated to be 0.028 K, 4 kPa, 0.6%, and 0.4%, respectively.