To reduce the expected increase in global mean temperature, the substitution of R134a as working fluid of refrigeration and air conditioning systems using refrigerants with the lowest carbon footprint possible is a priority. In this paper, the use of R1234ze(E) as pure fluid and mixed with R134a (resulting R450A) is assessed from a thermodynamic point of view. Then, these results are used to support experimental data obtained from a vapor compression system at three evaporating temperatures combined with other condensing temperatures. Regarding refrigerant main thermodynamic properties, R450A is very similar to R134a, even though the HFO R1234ze(E) is the main component of the mixture (58% compared to 42%). For both alternatives, the major differences with R134a are observed in relative molar mass, vapor density and liquid thermal conductivity. Compared with R134a, average cooling capacity obtained is lower for both alternatives (until 34% for R1234ze(E)) due to a reduction in suction density and refrigerating effect. Resulting COP of R1234ze(E) is also slightly lower even though specific compression work is significantly lower. Average R450A COP can match that of R134a at the same compression pressure ratios. Attending to the results of the drop-in tests, the main conclusion of the paper is that only R450A can be used as drop-in replacement if light reduction in system cooling capacity can be accepted.