The transformation of titanium silicide from the C49 to the C54 structure was studied using x‐ray diffraction of samples containing arrays of narrow lines of preformed C49 TiSi2. Using a synchrotron x‐ray source, diffraction patterns were collected at 1.5–2 °C intervals during sample heating at rates of 3 or 20 °C/s to temperatures of 1000–1100 °C. The results show a monotonic increase in the C54 transition temperature by as much as 180 °C with a decreasing linewidth from 1.0 to 0.1 &mgr;m. Also observed is a monotonic increase in (040) preferred orientation of the C54 phase with decreasing linewidth. The results demonstrate the power ofinsitux‐ray diffraction of narrow line arrays as a tool to study finite size effects in thin‐film reactions. ©1995 American Institute of Physics.