Aluminium powder of 99.7wt% purity and in the nominal particle size range –75+15µm has been sprayed onto a range of substrates by cold gas dynamic spraying (cold spraying). The substrates examined include metals with a range of hardness, polymers and ceramics. The substrate surfaces had very low roughness before deposition of aluminium in an attempt to separate effects of mechanical bonding from other forms of bonding. The cross-sectional area of a single track of aluminium sprayed onto the substrate was taken as a measure of the ease of initiation of deposition, assuming that once a coating had begun to deposit onto a substrate, its growth would occur at a constant rate regardless of substrate type. It has been shown that initiation of deposition depends critically upon substrate type. For metals where initiation was not easy, then small aluminium particles were seen to be deposited preferentially to large ones (due to their higher impact velocities); these may have acted as an interlayer to promote further building of the coating. A number of phenomena have been observed following spraying onto various substrates, such as substrate melting, substrate and particle deformation and evidence for the formation of a metal-jet (akin to that seen in explosive welding). Such phenomena have been related to the processes occurring during impact of the particles on the substrate. Generally, initiation of aluminium deposition was seen to be poor for non-metallic materials (where no metallic bonding between the particle and substrate was possible) and for very soft metals (in the case of tin, melting of the substrate was observed). Metallic substrates harder than the aluminium particles generally promoted deposition, although deposition onto aluminium alloy was difficult due to the presence of a tenacious oxide layer. Initiation was seen to be rapid on hard metallic substrates, even when deformation of the substrate was not visible.