The research is focused on evaluation of substrates not commonly used for biogas production and the development and optimization of processes adjusted to these substrates. This study deals with evaluation of sea weeds (Saccharina Lattisima and Laminaria digitata). Biomethane potential tests (BMP) have shown the methane potential of the algae to be 180-440 l CH4/kg organic material. These potentials are in the same range as potentials found for commonly used substrates such as sewage sludge and slaughterhouse waste. Sampling of produced biogas, substrate and digest were performed by using Solid Phase Microextraction (SPME) followed by analysis by a Gas Chromatograph with a Mass Spectrometrer (GC-MS) in order to develop a method to be able to characterize, monitor and possibly control the process.