A reservoir fluid on it's way from downhole to the surface tank is exposed to large changes in temperature and pressure. To know the phase behavior of the hydrocarbon mixture in every point on it's path is essential for recovery efficiency, beginning from well planning, reservoir simulation and up to the dimensioning of facilities. To get this data, expensive and time consuming analysis in a PVT (pressure, volume, temperature) laboratory is necessary. Today good mathematical models exist that can predict phase behavior in accordance with experimental data and are available as commercial software. This Master Thesis has the objective to develop a free software tool, that can predict realistic phase behavior of mixtures of up to three components and allows the user to perform basic PVT experiments. A main concern is an understandable presentation of the topic in order to be attractive for users who are not familiar with phase behavior yet. The application therefore can be launched from within the institute's homepage to be as convenient accessible as possible. Main concern in a professional point of view is the formulation and preparation of the calculations to be suited for direct implementation in a computer program. These procedures are so called VLE (vapor-liquid equilibrium), also known as Two-Phase Split calculations, in combination with an EOS (equation of state), in case of this thesis Peng-Robinson, to model the phase behavior. The thesis shows the used equations and algorithms as well as a comparison of results with literature and a commercial software.