In this study, we report on the structural and hyperfine properties of Al-doped SnO2 nanoparticles synthesized by a polymer precursor method. The x-ray diffraction data analysis carried out using the Rietveld refinement method shows the formation of only rutile-type structures in all samples, with decreasing of the mean crystallite size as the Al content. A systematic study of the unit cell, as well as the vicinity of the interstitial position show strong evidence of two doping regimes in the rutile-type structure of SnO2. Below 7.5mol% doping a dominant substitutional solution of Al+3 and Sn4+-ions is determined. However, the occupation of both substitutional and interstitial sites is determined above 7.5mol% doping. These findings are in good agreement with theoretical ab initio calculations.