Abstract <p>The effect of a preparation containing magnesium and phosphorus compounds on the composition of the resulting effluent and biogas productivity of litter-free chicken manure during its anaerobic monofermentation was studied. It was found that the introduction of this preparation into the bioreactor helps to decrease the pH of the medium, bringing it closer to the standard (7.0–7.5). The best result in biogas productivity among the options using different amounts of the preparation was obtained when it was introduced at a dose of 13.33 g/L of the bioreactor contents. The specific biogas yield in this case was 175.23 ± 5.69 L/g of organic matter (1.51% higher than in the control), and the specific methane yield was 68.38 ± 0.45 L/g (15.50% lower than in the control); however, the difference with the control option was insignificant, indicating the absence of a significant effect of this dose on the indicators of productivity. At the same time, the use of the preparation contributes to an increase in the concentrations of magnesium and total phosphorus in the effluent (by 1.20–1.27 and 1.21–1.27 times, respectively) and a decrease in the content of total nitrogen (by 1.45–1.71 times). In this case, magnesium and phosphorus are concentrated in the solid fraction, and nitrogen is concentrated in the liquid fraction, which is important when using the separated effluent as a fertilizer; an exception is the option with the use of the preparation at a dose of 13.33 g/L; here the content of total phosphorus was higher in the liquid fraction. In the tested doses (53.32, 26.66, and 13.33 g/L of the bioreactor contents), the preparation does not provoke the appearance of struvite crystals, which is favorable when operating the most common vertical bioreactors.</p>