This paper focuses on the characterization and optimization of microwave power transistors using a commercial on-wafer hannonic load. pull system. Specific attention is paid to the output tuning of the second harmonic impedance prese nted to the device. The ability to quantify the level of accuracy in a load pull system is IXplored by using various c alibration validation methods. Experiments and simulation comparisons are described for a GaAs pHEMT and a GaAs HJFET. The measured harmonic load pull data pointed to different guidance on how one would match the 2"d harmonic for best performance.