The ongoing environmental regulations aimed at cutting the carbon emissions are pushing the air conditioning and refrigeration industry to the exploration of refrigerants having lower and lower values of Global Warming Potentials. Many candidates have been proposed to replace the widely used R134a. Among the proposed alternatives, R450A is a near-azeotropic mixture made of R1234ze(E) and R134a (0.58/0.42 by mass, respectively), and R515B is an azeotropic mixture made of R1234ze(E) and R227ea (0.911/0.0899 by mass, respectively). The flow boiling behavior of these two refrigerants is not deeply explored in the open literature. In this context, this paper explores the flow boiling characteristics of R450A and R515B inside a compact microfin tube with an outer diameter of 5.0 mm. The tested microfin tube has an inner diameter at the fin tip of 4.28 mm, it presents 54 fins, with a fin height of 0.15 mm and a helix angle of 30°. The area enhancement ratio, i.e., the ratio between the effective heat transfer area of the microfin tube and the area of an equivalent smooth tube with the same fin tip diameter, is 2.1. The microfin tube is heated by hot water flowing inside a smooth copper tube wrapped around the test tube. The gap between the microfin tube and the smooth tube was filled with an alloy of tin/lead. Flow boiling tests were carried out for mass velocities ranging from 100 to 600 kg m^-2 s^-1, heat fluxes of 20, 40 and 60 kW m^-2, with a saturation temperature at the inlet of the test section of 30 °C. The experimental tests permitted to obtain the heat transfer coefficients and frictional pressure drops, highlighting the effects of the working conditions on the aforementioned outputs. R515B showed higher heat transfer coefficients, due to its more favorable convective properties. On the other hand, R515B showed higher values of pressure drops. Finally, the accuracy of some empirical correlations, available from the literature, has been verified for the prediction of the heat transfer coefficient and pressure drop during flow boiling inside the proposed enhanced tube.