The role of the sputtering parameters on the properties of thin A12O3 and A1N insulator layers for spin dependent tunneling junctions was studied. A12O3 insulators were prepared by rf sputtering from an A12O3 target in an Ar or Ar-O2 atmosphere and also by plasma oxidation in an O2 atmosphere of a thin Al layer. The thin Al layer was deposited by rf sputtering at room temperature. A1N was prepared by reactive sputtering from an Al target in a Ar-N2 atmosphere. The index of refraction n measured by ellipsometry, was used to optimize the sputtering parameters, and chemical etch rates were used for information on pin-hole density. The aimed values were n=1.75 for A12O3 (corundum structure) and n=2.2 for A1N. For reactively sputtered A12O3, under optimized deposition conditions, n reaches 1.65 for films 300 Å thick. A12O3 thin layers fabricated by plasma oxidation of a thin Al film were also prepared but the oxide layer was too thin for ellipsometry characterization. A1N insulating films show a constant n value of 1.92 from 400 Å down to 60 Å thick films. Rutherford backscattering analysis show that the ratio of Ar to N2 must be less than 1 to avoid O2 incorporation in the film. Junctions of the type glass/buffer/FM/insulator/FM, where FM is either Co or NiFe, were made by contact metal masks with a junction area of 0.25 mm.2 The A12O3 deposited by sputtering from an A12O3 target under conditions expected to give n>1.5 had a magnetoresistance at room temperature MR(RT) of 2.5% with a junction resistance Rj of 2.2 m Ω and by fitting the I-V curve to existing tunneling theory gave an effective barrier thicknesst of 20 Å and a barrier heightΦ of 2.2 eV. These occurred in only one sample (9 junctions) and were not reproducible. The A12O3 insulators prepared by plasma oxidation of a thin Al layer gave a MR(RT)= 4%, tj = 22 Å, Rj=250 k,Ω and Φ =1.4 eV. The A1N insulator with n =1.9 gave a MR(RT)= 1.6 %, t = 20 A, Rj = 4.8 kΦ and barrier height 1.2 eV. In the last two cases junctions with MR at room temperature can be produced with reasonable reproducibility, Al2O3 samples showed a 15–20% increase in MR as the temperature was lowered to 17 K, while AIN junctions show an increase of a factor of 2 in MR at 77 K.