The isochoric specific heat capacity (c(V)) of trans-1,3,3,3-tetrafluoropropene (HFO-1234ze(E)) and mole fraction x = 0.489 HFO-1234ze(E) + (1 - x) = 0.511 CO2, in the liquid phase were measured with a twin-cell type adiabatic calorimeter. The sample purity of HFO-1234ze(E) and CO2 was certified to have a minimum purity of 0.9996 mole fraction and 0.99999 mole fraction respectively by gas chromatographic analysis. The measurements were obtained for temperatures ranging from (270 to 425) K and at pressures up to 30 MPa. Temperatures were measured with a platinum resistance thermometer on the bottom of each cell and were reported based on the International Temperature Scale of 1990 (ITS-90). Sample pressures were measured with a quartz crystal transducer. Densities were calculated from the volume of the calorimeter cell and the sample mass. The expanded uncertainty (with a coverage factor k = 2) of temperature measurements is 13 mK and 8 kPa for pressure measurements. The expanded relative uncertainty of density is 0.16 %. As a result, the experimental expanded relative uncertainty of the isochoric specific heat capacity (c(V)) is estimated to be from 3.8 % to 4.6 % in the liquid phase. The c(V) measurements were almost the same as the values calculated from the proposed equations of state except near the critical isochore.