Speed of sound measurements in high purity samples of two novel candidate refrigerants, 2,3,3,3-tetrafluoropropene (R-1234yf) and trans-1,3,3,3-tetrafluoropropene (R-1234ze(E)), are reported along five isotherms in the temperature range of (260 to 360) K and for pressures up to 10 MPa. The experimental technique is based on a double-reflector pulse-echo overlap method. The acoustic path lengths were obtained by a comparison with measurements in pure water carried out at atmospheric conditions. The speed of sound experimental results are characterized by an overall estimated uncertainty of less than of 0.1 %.