Ozonation of C.I. 18735 Mordant Red 19 (5-hydroxy-3-methyl-1-phenyl-4[5-chloro-2-hydroxy-3-sulfophenyl)- azo]-pyrazole, sodium salt, HFANS (1,2-hydroxo phenyl azo,8-amino naphthalene,3,6-disulfonic acid mono sodium salt) and their metal complexes with chromium, C.I. 18736 Acid Red 180 and HFANS-Cr, has been carried out in aqueous medium. Various parameters, such as ozone consumption and colour elimination percentage, were investigated. The biodegradability of these dyes was determined and supported with the standard method for methylene blue active substances. The metal complexes of the mono-azo dyes consumed more ozone than that without metal. Finally, the differences between ozonation of azo dyes and their metal complexes were outlined, based on the experimental results obtained.