Local flow boiling heat transfer coefficients were measured for R-134a and R- 123 evaporating inside a microfin tube for horizontal flow and vertical upflow over a wide range of mass velocities, vapor qualities and heat flux. The objective was to study the effect of tube orientation on microfin heat transfer, primarly to gain more insight into how microfin tubes augment heat transfer. In addition microfin heat transfer augmentation relative to previously measured plain tube test data was investigated. The experiments showed a significant effect of flow direction on local microfin heat transfer coefficients with horizontal flow surprisingly giving better heat transfer than upflow. In addition, the relative level of augmentation increased significantly with local vapor quality.