The diffusion of nitrogen in aluminum is investigated for three different methods of nitrogen implantation. Low energy nitrogen implantation (LEI) at 1 keV and plasma immersion ion implantation (PIII) with -30 kV pulses were performed in the temperature range from 250 to 500 degreesC, together with conventional ion implantation at room temperature with 100 keV N+ ions and subsequent annealing in vacuum at 440 or 485 degreesC. For the LEI samples a rather high diffusion leading to AIN layers of a few microns in less than 1 h was found, whereas a low diffusion coefficient was obtained for the PIII samples, exhibiting a sharp stop near 350 degreesC. In contrast no diffusion was measured in the implanted and annealed samples. Several different factors are proposed to influence the observed diffusion: A competing path between the formation of AIN by trapping of N and N diffusion is responsible for the dependence of the layer thickness on the supply of nitrogen. The oxygen contamination drastically reduces the diffusion constant by occupying the surface sites and blocking the diffusion path. Finally, the phase transformation from c-AIN to h-AIN may cause the sudden onset of diffusion around 350 degreesC.