Chemical additives of magnesium chloride (MaC) and ferrous sulfate (FS) were investigated in sewage sludge composting. A treatment without chemical additive was used as the control group (CK). The effects of MaC and FS on ammonia (NH3) and greenhouse gases (CO2, N2O and CH4) emissions were evaluated. The results showed that the addition of MaC and FS promoted the degradation of organic matter and increased the ammonium concentration of the compost product. The maximum pH values decreased from 8.98 (CK) to 8.49 and 8.0 in MaC and FS treatments, respectively. In addition, the NH3 emission for the MaC and FS treatments reduced by 58.3 % and 82.9 %, respectively, whereas the methane (CH4) emission decreased by 22.9 % and 24.9 %, respectively. However, in FS treatment, the carbon dioxide (CO2) and nitrous oxide (N2O) emissions were higher than those in CK. Furthermore, the FS treatment exhibited lower germination index values (71.4 %) and higher electrical conductivity (6.62 mS/cm). Therefore, MaC is suggested as an additive for reducing greenhouse gas emission and conserving nitrogen during sewage sludge composting. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.