•The firing temperature of sewage sludge ash (SSA) affects the brick properties.•The best results were obtained with mixed-in SSA fired at 900 °C.•The soluble salts level rises with SSA addition but remains within allowed limits.•Higher firing temperatures reduce the soluble salts level in the bricks. The influence of combustion temperature on the properties of sewage sludge ash (SSA) as a substitute material for the production of fired clay bricks was studied. The experiments were performed with sewage sludge (SS) from wastewater treatment plants (WWTP) in Zagreb (ZG) and Karlovac (KA). SS samples were fired at temperatures of 800, 900 and 1000 °C, and the properties of the resulting SSA were determined and compared with clay and SS. Bricks with 5 wt% SSA as clay substitute were prepared and compared with clay bricks (control). SSA bricks produced at higher temperatures are of higher strengths and lower water absorption, which are both desirable properties. Compressive strength tests showed that SSA obtained at a firing temperature of 900 °C is the most suitable for use in bricks. For the first time, the influence of SSA on the content of soluble salts in bricks was tested. The content of soluble salts, expressed by the electrical conductivity of leachate, in SSA bricks showed increased values compared to the control bricks, which were 2.8 and 4.6 times higher in bricks with SSA produced at 800 °C. Still, bricks with SSA prepared at higher temperatures had lower content of soluble salts and also SSA from KA had lower soluble salts content than SSA from ZG. The total amount of Na+, K+, Mg2+ in all bricks made with SSA was below the limits for category S2 according to EN 771-1.