Thermochemical calculations were undertaken for a series of pack powder mixtures for codepositing A1 and Si to form diffusion coatings on gamma-TiAl by the pack cementation process. The results of calculations indicated that codeposition of A1 and Si on gamma-TiAl by the pack cementation process is possible using NH4Cl- and AlCl3-activated packs, containing elemental A1 and Si as the deposition source. To achieve conditions favourable for codeposition, the pack Al content should not be higher than 2wt.%. Coating deposition experiments were also carried out and it was demonstrated that codeposition of A1 and Si on gamma-TiAl could be achieved at 1100 C using the AlCl3-activated packs. The coatings obtained had a multiple layer structure, consisting of an outer silicide layer, an inner TiAl3 layer, and a diffusion zone at the boundary between the coating and the substrate. It was suggested that the coating was formed via a sequential deposition mechanism through inward diffusion of A1 and Si. The conditions for codepositing A1 and Si from the vapour phase to form silicide and aluminide diffusion coatings on gamma-TiAl with a coherent structure free from microcracking and spallation by the pack cementation process were discussed. The oxidation resistance of the coating was evaluated in air by intermittently monitoring the weight change at room temperature. The results demonstrated that the coating is thermally stable and can provide effective protection against oxidation for gamma-TiAl at temperatures up to 850degreesC. (C) 2003 Elsevier Science B.V. All rights reserved.