The dyeing behavior of selected both carbocyclic and heterocyclic vat dyes on polyamide PA and polyester PET fabrics have been examined. Factors affecting dyeability such as sodium hydrosulphite concentration, dyeing temperature and dye concentration were also studied. It was found that color strength (K/S) values of the carbocyclic vat dyes on PA and PET are high, which confirms the usefulness of these dyes for dyeing PA and PET fabrics in a neutral medium (acid leuco) at the specific temperature 120°C with very good fastness properties. However, K/S values obtained using heterocyclic vat dyes are very low compared with those obtained using carbocyclic vat dyes. The results indicate the possibility of using carbocyclic vat dyes for the one-bath and one-stage dyeing of PA/PET blend fabric with very good fastness properties.