K17[Ln(As2W17O61)2] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Tm, Yb) were prepared The stability range of the anions is given in terms of the pH. Their UV, IR, DTA, x-ray powder diffraction patterns, XPS, magnetic susceptibility of the lanthanides in complexes and polarog. reduction studies are reported. Magnetic measurement confirms that the lanthanide elements exist mainly in +3 oxidation state in these complexes and the magnetic moments are very close to the value given by van Vleck. XPS determination represents Ln-O bond of coordination character as well as the shifts of W 4f and O 1s between K17[Ln(As2W17O61)2] and (NH4)6As2W18O62. Polarog. determination shows that there are 3 2-electron waves when these complexes are reduced indicating the reduction of W.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, INFRARED SPECTROPHOTOMETRY, HETEROPOLYACID, X-RAY DIFFRACTION ANALYSIS, THULIUM, LANTHANON, POLAROGRAPHIC ANALYSIS, POTASSIUM COMPOUNDS, X-RAY PHOTOELECTRON SPECTROMETRY, CERIUM