It is proposed that very low levels of solid solubility may be due to dynamic diffusion properties rather than thermodynamic equilibrium. In such systems, retrograde solid solubility will be observed at certain temperatures because of the thermal activation of an efficient solute diffusion mechanism or to a peak in the concentration of an emission-inducing defect. For the particular case of bismuth in copper we show that a likely cause of the solubility peak is the activation of thermal glide of dislocations.