Two different and accurate methods are described for the determination of Sc+3++. Y3+, , La3+, and UO22+. The spectrophotometric and polarographic methods are based on the formation of complexes using 1`-(2-arsono-phenylazo)2`-naphthol (I) and its 3`-carboxylic acid (II)are used. The coloured products are quantified spectrophotometrically and polarographically. The optimization of different experimental conditions is described. The stoichiometric ratio and stability constant of the formed complexes are studied. The molar absorptivity and Sandell sensitivity are also calculated. Comparison between the results obtained indicaited that spectrophotometric method is more sensitive thean the polarographic one for these compounds.