Densities of sputtered atoms in Ar rf magnetron discharges have been measured by optical absorption. The resonant and self-absorption methods have been used. The first one has given Al and Mg atom densities with an uncertainty of 40% and the second one the order of magnitude of Si atom densities. The Al atom densities are strongly decreasing from 93.7% Al alloy target: (1-5) x 10(11) cm(-3)-10.3% Al aluminosilicate glass: (1-2) x 10(9) cm(-3). The Si atom density is in the range (3-9) x 10(10) cm(-3) in the aluminosilicate glasses. The Al and Si sputtered atom densities follow the sputtering yields of Al metal and Al, Si oxide targets.