The importance of the balance between the nutrients in the anaerobic digestion process was assessed at laboratory scale. A soluble synthetic solution was used as substrate. The variable used to monitor the proportion of nutrients in the substrate was the [C]/[N + P] ratio expressed as equivalents of each element. A blank assay with a constant ratio of 53 and three experiments in which this variable was modified in the range 53–1 were performed. An increase in the concentration of nitrogen and phosphorus both separately and in combination was observed. The most relevant results show a loss of N in the biogas in the form of NH3 for concentrations of around 400 mg N soluble/L in the liquid phase of the reaction, and a loss of soluble phosphorus in the form of calcium phosphate due to precipitation at concentrations above 250 mg P soluble/L, regardless of the experimental procedure. No significant effects were observed in the production of biogas for a ratio in the range 53–40, while biogas production decreased by 8% for a ratio of 20 when both N and P increased in the best of cases and by up to 46% when only N increased. © 2018 American Institute of Chemical Engineers Environ Prog, 38: 228–236, 2019