When ferrous Fe combines with aa''-dipyridyl a red dye results which can be estimated colorimetrically. Amts. of Fe as small as 0.2 [gamma]/ml. can be estimated when treated with 0.4 ml. of 1% aa[image]-dipyridyl soln. in 0.1 N HC1.; 0.5 ml. of 20% soln. of Na2SO3 7H2O is added to reduce ferric Fe to ferrous. The absorption spectrum of the dye is given and complementary filters suggested. A wet ashing process is described for use where necessary.