Precipitation of Ti5Si3 (ζ) during tempering of quenched Ti-2wt% Si alloys has been studied using transmission electron microscopy. Two forms of the suicide are observed, each with a discrete orientation relationship with the matrix phase (α′)- At high temperatures (T > 973 K), the hexagonal precipitate phase shares a 2-fold axis of symmetry [101̄0] with the matrix and takes the form of rods of square section. The rod axis [1̄21̄3]ζ is nearly parallel to [1̄21̄0]α and remains approximately invariant during transformation. Six crystallographic variants of the precipitate phase may be distinguished. At lower temperatures (T = 773-873 K), the precipitate has the form of hexagonal-shaped plates lying parallel to the matrix basal plane. Parent and product lattices share a common 6-fold axis and the rotation of the precipitate lattice about this axis is such as to generate two sets of parallel directions of low lattice misfit, 〈1̄21̄0>ζ//〈4̄51̄0〉α′ and 〈101̄0〉ζ//〈213̄0〉α′, in the parallel basal planes. The former set defines the directions parallel to the facets of the hexagonal plates, and the latter set directions normal to them. The two crystal forms are found to be compatible with the symmetry of the intersection point groups defined by the respective orientation relationships and composed of symmetry elements preserved during transformation.