Hydro-mechanical transmissions attempt to balance the productivity increases of hydrostatic transmissions with the higher efficiencies of mechanical transmissions. This technology provides a smooth transition between gear ratios useful to power vehicles of various applications. This balance is accomplished by splitting the input power into two phases, hydrostatic and mechanical transmission. These two phases are then added back together through the use of a planetary gear set. In this study, laboratory experiments were preformed to characterize efficiency and power flow of the hydrostatic and mechanical phases of the transmission. Comparisons of the performance of a hydro-mechanical transmission with the stand alone hydrostatic transmission were made. A mathematical model was also developed to simulate transmission performance which was compared with experimental results. Conceptually, higher efficiencies would be achieved when all of the power is transmitted mechanically, under the assumption of no hydrostatic leakage. The results show this to be true while identifying transmission ranges of maximum efficiency. By being able to model and understand the characteristics of the hydro-mechanical transmission, the design can be optimized to fit different applications. This work also shows that by adding a simple planetary gear set to a hydrostatic transmission, efficiencies and power ranges can both be increased.