In the biogas industry, feedstock plans are used to estimate methane production and nutrient content in the digestate, however, these predictions do not consider the mineralized nitrogen fraction of the feedstock, which is useful when determining the quality of the digestate. In this study, the artificial fiber bag technique, which is commonly used to study feedstock degradation in ruminants, was implemented in anaerobic digestion to quantify mineralization of N and S. The artificial fiber bags were used to enclose substrates but with access to inoculum because of small pores in the bags, thereby enabling digestion. The content of the bags was analyzed before and after digestion to quantify residual mass as well as N and S concentration in the substrate. The method was validated through batch anaerobic digestion of a single substrate with and without bags, where the bags showed little influence on methane production and degradation. Semi-continuous anaerobic digestion experiments showed higher substrate degradation and higher N and S release at thermophilic conditions using four different types of feedstocks and proved useful for solid feedstocks but less so for semi-solid feedstock. For N, most of the mineralization occurred during the first 15 days over a trial of 30 days.