The analysis of extracts from the South American plant Justicia pectoralis Jacq. permitted the identification, among other compounds, of coumarin, dihydrocoumarin, umbelliferone and 3-(2-hydroxyphenyl)propionic acid by gas chromatography/mass spectrometry (GC/MS); the acids and phenolic compounds were derivatized with diazomethane. GC/MS of simple coumarins, phenylpropionic acids and their hydroxylated isomers was performed after derivatization through methylation and trimethylsilylation; these results may be useful for the identification and quantification of these compounds in other biological materials.