We provide direct evidence of the dependence of the critical concentration of IV-type atoms at the order–disorder transition in ternary metastable (III–V)–IV, zincblende–diamond semiconductor alloys on the growth direction. The excellent agreement between the experimental and model predicted critical concentrations is evidence that the atomic ordering in these alloys is ruled almost entirely by substrate geometry. In this letter, we report the observation of the critical concentration dependence on substrate orientation in (GaAs)1−x(Ge2)x metastable alloys, epitaxially grown on (001), (111), (112), and (113) oriented GaAs. A different long-range order parameter behavior with Ge concentration is obtained for each growth direction. The Ge critical concentrations observed are 0.36, 0.96, 0.59, and 0.50±0.03, for the orientations listed above, values that compare well with those obtained from a Monte Carlo simulation of the growth, 0.33, 1.0, 0.64, and 0.54, respectively. © 2000 American Institute of Physics.