Measurements of the speed of sound in gaseous cis-1,3,3,3-tetrafluoroprop-1-ene, (R1234ze(Z)), are pre- sented. The measurements were performed using a quasi-spherical acoustic resonator at temperatures between 307 K and 420 K and pressures up to 1.8 MPa. Ideal-gas heat capacities and acoustic virial coeffi- cients over the same temperature range were directly calculated from the results. The relative accuracy of our determinations of the speed of sound w ( p,T ) of R1234ze(Z) was approximately ±0.02%. The accuracy of the determination of the ideal gas heat capacity ratio γ0 ( T ) was approximately ±0.25%. These data were found to be mostly consistent with the predictions of a fundamental equation of state of R1234ze(Z).