Conditions were developed for the determination of ascorbic acid using tris(4,7-dihydroxy-1,10-phenanthroline) iron(III) as reagent. The method was applied to the determination of ascorbic acid in pure form and in pharmaceutical preparations. The method is sensitive (0.20 to 3.6 μg/mL of ascorbic acid) and rapid, and tolerates the presence of common ingredients.