AbstractOxygen‐contaminated, melt‐spun, binary Ti‐Si alloys have been examined by using transmission electron microscopy. The microstructure of alloys in the range of 4 to 10% Si (by weight) are cellular and consist primarily of α‐Ti and the silicide Ti5Si3.Contained only within the Ti5Si3regions are small, approximately spherical particles which are ⩽ 10 nm in diameter. Due to their small size, the crystal structure of these particles could not be determined by using conventional diffraction techniques such as Selected Area or Convergent Beam Diffraction. By conducting a number of tilting experiments and observing the moire fringe patterns produced when various matrixTi5Si3planes were used to image the sample, the crystal structure of the particles and the orientation relationship which exists between them and the matrix were deduced. The unknown particles, termed the Z phase, were found to be hexagonal with slightly different lattice parameters from the matrix Ti5Si3.Their relationship with the matrix was such that they appeared to be totally coherent. This may indicate that Z is an oxide based on the intermetal