The interaction of various substrate analogues with 5-aminolaevulinic acid dehydratase (porphobilinogen synthase) from Bacillus subtilis is studied kinetically and by electrospray mass spectrometry; 5-chlorolaevulinic acid is shown to be a non-specific alkylating agent but 5-amino-3-thialaevulinic acid is a potent mechanism-based inactivator.