An on-line HPLC-1,1-diphenyl-2-picrylhydrazyl (DPPH*) method has been improved for the detection of polar and nonpolar radical scavenging compounds from complex plant extracts. Eight water extracts were prepared from steam-distilled essential oil-extracted Lamiaceae plants (Origanum vulgare L., O. onites L., O. minutiflorum O. Schwartz et P. H. Davis, O. syriacum L., Satureja cuneifolia Ten., Thymbra spicata L., Coridothymus capitatus (L.) Reichb. f., Majorana hortensis Moench). After the components within each extract had been separated by reverse phase chromatography using 10% to 100% methanol with 2% acetic acid as a mobile phase, analytes capable of scavenging a citric acid-sodium citrate buffered methanolic DPPH* solution were detected by post-column derivatization at 517 nm. The HPLC-DPPH* on-line method was applied to the qualitative and quantitative analysis of these Lamiaceae plant extracts. There was a strong correlation between the scavenging (negative) peak area and the concentration of the radical scavenging reference substances used. The radical scavenging compounds within the extracts were determined as benzoic acid and hydroxycinnamic acid derivatives, flavonoids and diterpenoids according to their retention time and UV spectral data. Rosmarinic acid and carnosic acid were identified as the dominant radical scavengers in these extracts by this method.