Solid state double resonance NMR experiments on phosphorus-impregnated gamma-Al2O3 and amorphous AlPO4 have been conducted to investigate the interaction between the impregnated phosphorus and the gamma-Al2O3 surface. The P-31-Al-27 REDOR and TRAPDOR experiments have shown that most phosphorus is in close contact with aluminum, thereby excluding the possibility that stacked phosphate layers or bulk phosphates are formed when more phosphorus is impregnated than is necessary to cover the entire gamma-Al2O3 surface. The Al-27-P-31 experiments enabled the Al-27 spectrum of the aluminum atoms which interact with phosphorus to be separated from the spectrum of the bulk gamma-Al2O3. These double resonance experiments have shown that a layer of AlPO4 is indeed formed on the gamma-Al2O3 surface and that the structure of this layer, although similar, has a slightly higher degree of-ordering than the structure of amorphous AlPO4.