The surface relief effect associated with the formation of h.c.p. γ (or γ') plates in the f.c.c. α matrix of an Al-15 w/o Ag alloy at 400°C is primarily a single planar tilt which can be described as the product of an invariant plane strain transformation with an appreciable “shear” component. The interferometrically measured “shear” extrapolates to the theoretical, or half-twinning “shear” at a time corresponding to the beginning of transformation. However, silver X-ray scanning studies show that while still growing, the precipitate plates contain more Ag than the α matrix. Thus there cannot be a lattice correspondence between γ and α. It has been previously shown that γ plates in this alloy thicken solely by the ledge mechanism (in the form of Shockley partial misfit dislocations which are also able to serve as ledges) and that the lateral growth rate of these ledges is controlled by the long-range volume interdiffusion of Al and Ag. (12) The combination of these results and the silver X-ray studies demonstrates that the unit atomic mechanism of growth is one of diffusional jumps, rather than of shear. The surface tilt was found to decay continuously with time in a manner which indicates that it does so by an untilting process. This is explained in terms of the loss of a proportion of the misfit dislocations (as ledges) to the free surface preferentially from that broad face of a plate which originally produced the tilt. Elastic “shear” strain energy accompanying the formation of a γ plate provides the driving force for the decay process. (2)