Semi-automatic catalytic titrations of N-penicillamine, N-acetylcysteine, cysteine and 2-mercapto-propionylglycine using silver nitrate as titrant are described. Procedures are based on the inhibitory effect of these thiol-containing drugs on the silver-catalysed phloxin-persulphate reaction. The end-point is determined spectrophotometrically by measuring the absorbance at 537 nm. The proposed methods were applied to the determination of the drugs in pharmaceutical preparations.