Composting of poultry manure (initial C:N ratio - 11.3) in a thermostatically aerated pile resulted with overheating (>65°C) and rapid loss of Total Volatile Solids (TVS) and of nitrogen. Adding five percent (on a dry weight basis) of squeezed grapefruit peels (initial C:N ratio of the mixture - 12.4) lowered the pH of the aqueous phase of the raw materials from 6.6 to 5.8 (in a 1:10 extract). This enabled controlling the pile temperature below 60°C and increased the amount of conserved N by ca. 80 percent. Nitrogen was conserved more than the TVS on a relative basis in this mixture but conserved less than the TVS in the poultry manure alone. This suggests that previously released NH₄⁺ was biologically immobilized in the mixture. Although the effect of the added acidic carbon source was apparent until the end of the composting period on the above mentioned parameters, a sharp rise in pH and a decline in C:N ratio occurred after less than 12 days and less than 39 days, respectively. This suggests that adding more squeezed grapefruit during the composting period may have caused additional beneficial effect on the characteristics of the end product.