Vicinal Si(001) surfaces are believed to undergo a phase transition between single and double atomic height steps as either temperature or angle of miscut is varied. Here we calculate the full temperature-angle phase diagram, which is found to be quite different than previously believed. In particular, there is a critical point above which there is no phase transition at all. The results appear to explain the rather continuous behavior seen in a variety of experiments. © 1991 The American Physical Society.