We have used photoelectron spectroscopy to observe thermal isomerization in cesium-halide cluster anions. In many of the (CsX)(n)Cs-m(-) (X = Cl, Br, I; n = 2-7 ; m = 0, 1) systems we have studied, small changes in the source nozzle temperature produce dramatic changes in the distributions of cluster isomers. There is also evidence that the isomer distributions are in dynamic equilibrium, with their temperatures controlling the rates and probabilities of isomerization processes. We present data and discussion of several cluster families that exhibit this behavior.