The influence of the wastewater composition on the denitrification and biological P-elimination in the Sorption Denitrification P-eliminations Process (S-DN-P-process) was examined. Batch type experiments were performed to examine the influence of various substrates. Among the three different substrates prepared, only 11% COD(filt) was taken up from starch containing wastewater (starch preparation). Acetate is an easily degradable substrate for the biological P-elimination, and showed ca. 30% more acetate was taken up compared to the raw wastewater (wastewater preparation). Starch is a barely degradable substance, because it must be hydrolysed before digestion. Starch initially sorbs slowly (approximately 30 min), and may be hydrolysed during this time. During the investigations, the biological P-elimination and the denitrification were found to be dependent on the wastewater composition. The P-elimination rate was determined to be 38% with the acetate preparation, while a P-elimination of 6% was obtained with the starch preparation. The wastewater preparation showed a P-elimination of 56%, the value of which was almost 18% more than the acetate preparation. The biological P-elimination and denitrification depend not only on the dissolved parts in the wastewater, but also on the undissolved content.