The atomic absorption behavior of europium and ytterbium was studied in an air-hydrogen flame as a cool flame. The effects of inorganic acids in the range of 0.01 to 2.5 M on the absorption of europium (300 ppM) and ytterbium (30 ppM) were investigated. The effects of metal ions in the range of l0 to 5000 ppM on the absorption of eurcpium and ytterbium were investigated. The absorption of europium increased in the presence of potassium of 10 to l000 ppM, and that of ytterbium decreased with increasing the concentration of potassium. Magnesium and aluminum remarkably suppressed the absorption of both the elements, but the interferences with the absorption of europium were eliminated by adding l% lanthanum. The absorption of europium gradually decreased with increasing the concentration of calcium, and that of ytterbium decreased by 80% in the presence of calcium of 100 to 5000 ppM. The absorption of both the elements decreased with increasing the concentration of Cr, Mn, Fc, and Pb. The absorption of europium increased by 20% in the presence of La and Nd more than l00 ppM, and by 50% in the presence of Sm of l000 ppM, but that decreased with increasing the concentration ofmore » Y and DY,. The absorption of ytterbium decreased with increasing the concentration of Y, La, Eu, Dy, and Er. The calibration curve for europium was linear in the range of 0 to 400 ppM. The calibration curve obtained by adding 0.l of 0 to 400 acid showed a sensitivity of 2.7 times in comparison with that for europium alone. The calibration durve for ytterbium obtained by adding 0.1 M perchloric acid showed a sensitivity of 4.5 times that for ytterbium alone and of 1.8 times that for yettebium obtained in a nitrous oxide-acetylene flame, Europium in synthetic or practical samples of rare earth oxide was determined by atomic absorption method using the air-hydrogen flame. A sample was dissolved by perchloric acid, and europium in the sample solution was determined. (auth)« less