This paper reports optimal conditions for the reaction of aluminum (III) with chromeazophenol KS [also called trisodium 2-(2-hydroxy-3-carboxy-5-sulfobenzeneazo)-1, 8-dihydroxynaphthalene-3, 6-disulfonate, which will be abbreviated CAPKS] (Λmax= 585 nm, pH = 4, CR= 1.59 X 10−4M) and a new procedure developed for the spectrophotometric determination of exchangeable aluminum in soils.