The presented study investigates and compares the genotoxic potential of four commonly used diesters of phthalic acid (phthalates) in the presence and absence of S9 metabolic activation by using the SOS/umuC assay on Salmonella typhimurium TA1535 [pSK1002]. Phthalates are plasticizers with a wide range of applications for manufacturing plastic and non-plastic products. Therefore, phthalates are identified as contaminants in a variety of consumer products. The results showed that phthalates did not induce mutations without S9 metabolic activation (with the exception of BBzP). The genotoxic activity was further studied with the S9 metabolic activation. Assessment of the genotoxic potential indicates a significant (p<0,05 for DEP, DBP and BBzP) and highly significant (p<0,01 for DEHP) increase of this potential for each phthalate sample.