This paper presents the results of a study of the morphology of alumina splats deposited on stainless steel and alumina substrates. The substrates were either plasma sprayed or coated via plasma enhanced CVD. Substrates that were plasma sprayed were annealed if necessary to get specific phase structures, then polished to around 0.4 μm (Ra). CVD-coated substrates with an Ra ~6 nm and a columnar amorphous structure were sprayed as deposited. Splat studies show that the crystal structure of alumina substrates and the release of entrapped gas have a major influence on splat formation. For plasma sprayed coatings, disk-shaped splats with excellent adhesion properties were obtained on hot γ alumina, while on α alumina, splat shape and morphology were irregular and adhesion very poor. The effect of entrapped gas, on the other hand, can be seen in the splats that formed on the CVD-coated substrates. These splats were very porous and, in many, most of alumina flowed out to rim. As the paper explains, this is the result of gas release upon impact of molten particles, which reduces wettability and thermal contact between the splat and substrate.