Synchrotron radiation photoemission results for Sm/GaAs(110) interfaces formed and studied at 20 and 300 K show temperature dependencies that can be related to differences in surface growth structures and kinetic constraints. Submonolayer growth at 300 K produces two distinct ordered Sm chain configurations, as shown by scanning tunneling microscopy, and the photoemission results demonstrate that Sm atoms in these chains are divalent. These low‐surface‐density divalent configurations are precursors to surface disruption that, with additional Sm deposition, produce reacted clusters in which the Sm atoms are trivalent. Ultimately, Sm metal nucleation occurs on the reacted region and the overlayer thickens, with Ga and As atoms segregating to the surface region. For 20‐K growth, the valence‐band results show much slower conversion from divalent to trivalent Sm bonding, despite evidence that the amount of disruption is equivalent at 20 and 300 K. We attribute these differences, and those in the Ga and As core levels, to the freezing‐in of an amorphous Sm–Ga–As mixture at 20 K. Hence, kinetic factors curtail atom rearrangements that occur readily at 300 K. Annealing of thin overlayers to 300 K removes kinetic constraints and produces Ga, As, and Sm bonding that is spectroscopically equivalent to that observed for 300‐K growth. Sm/GaAs(110) interfaces formed by cluster assembly are shown to be unstable. Together, these results demonstrate that high‐atom‐density Sm contacts to GaAs are thermodynamically very unfavorable and that the instability generated by increasing the surface coverage provides the driving force for disruption.