Abstract: Mercaptopyrimidines (e.g., thiouracils and thiobarbituric acid) were determined by their kinetic catalytic effect upon the cited reaction. At the µg level such compounds could be determined in 50 mL of sample at pH 6 containing 1 g of NaN3 and 10 mL of 0.01 M iodine; after a 2-min induction period, 0.01 M NAASO2 was used for back-titration with starch as indicator. The operating range was typically from ~2 to between 15 and 30 µg of analyte. Determination at the ng level could be achieved by automatic titration with dead-stop end-point detection in 15 mL of solution containing 0.2 g of NaN3 and 4 mL of 0.1 M HCl. Direct titration was with 2.5 mM iodine in 0.2 M KI added at 10 µL min-1. A flow injection technique could also be used with a stream (1.4 mL min-1) of 50 µM-iodine containing 2% of NaN3 and 0.15 M KI at pH 5.9.