Some species of sulphate-reducing bacteria (SRB) are known to mediate the formation of dolomite and Mg-calcite. However, their exact role in the mineralization process remains elusive. Here, we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by SRB can occur in the absence of living cells, through passive mineralization of their exopolymeric substances (EPS). SRB capable of mediating dolomite were cultivated in the laboratory, allowing them to secrete EPS. Microbial activity within the cultures was subsequently inhibited with antibiotics. Only after this step, Ca2+ and Mg2+ were added to the solution and carbonate minerals could form. Mg-calcite and disordered Ca-dolomite precipitated in association with EPS. The mol.% of Mg2+ in the crystals increased with longer incubation times. This result demonstrates that organic compounds produced by SRB can mediate the formation of Ca-Mg carbonates in the absence of an active metabolism.