A key parameter for understanding and controling the anaerobic biogas process is the concentration of volatile fatty acids (VFA). However, this information has so far been limited to off-line measurements using labor-intensive methods. We have developed a new technique that has made it possible to monitor VFA online in one of the most difficult media: animal slurry or manure. A novel in situ filtration technique has made it possible to perform microfiltration inside a reactor system. This filter enables sampling from closed reactor systems without large-scale pumping and filters. Furthermore, due to its small size it can be placed in lab-scale reactors without disturbing the process. Using this filtration technique together with commercially available membrane filters we have constructed a VFA sensor system that can perform automatic analysis of animal slurry at a frequency as high as every 15 minutes. Reproducibility and recovery factors of the entire system have been determined. The VFA sensor has been tested for a period of more than 60 days with more than 1000 samples on both a full-scale biogas plant and lab-scale reactors. The measuring range covers specific measurements of acetate, propionate, iso-/n-butyrate and iso-/n-valerate ranging from 0.1 to 50 mM (6-3000 mg). The measuring range could readily be expanded to more components and both lower and higher concentrations if desired. In addition to the new VFA sensor system, test results from development and testing of the in situ filtration technique are being presented is this article.