Transformation behaviour in thin foils of a Ni-12 at.% Ti alloy was investigated by means ofin situaging and electron irradiation experiments inside a high voltage electron microscope. It is shown experimentally that the mode or the morphology of precipitation and ordering reactions in the thin foils differ somewhat from that observed in the bulk material. In the thicker part of the thin foils agedin situat 873–973 K, a periodic modulated structure is observed to consist of a periodic array of cuboidal coherent particles along the [100] crystallographic directions. The development of ordering within the solute enriched particles appears to be much slower than in the bulk specimen In the thinner part of the foil or in the near-surface regions, no precipitation or ordering occurs and so-called precipitate-free zones (PFZs) are observed. At a higher temperature of 1073 K, precipitation takes place preferentially at the foil surfaces Electron irradiation at elevated temperatures is found to disturb the formation and growth of a metastable modulated structure. and alter the distribution and the morphology of precipitate particles initially present.