The spin Hall eect converts charge current to pure spin currents in orthogonal directions in materials that have signicant spinorbit coupling. The eciency of the conversion is described by the spin Hall angle (SHA). The SHA can most readily be inferred by using the generated spin currents to excite or rotate the magnetization of ferromagnetic lms or nano-elements via spin-transfer torques. Some of the largest spin-torque-derived spin Hall angles (ST-SHA) have been reported in platinum. Here we show, using spin-torque ferromagnetic resonance measurements, that the transparency of the Ptferromagnet interface to the spin current plays a central role in determining the magnitude of the ST-SHA. We measure a much larger ST-SHA in Pt/cobalt (0.11) compared to Pt/permalloy (0.05) bilayers when the interfaces are assumed to be completely transparent.