Preservation of cells and tissues at low temperatures requires the presence of effective cryoprotectants with low toxicity to which cells are relatively permeable. Two similar compounds, dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO₂), exhibit both features for cryoprotectants, yet DMSO is a very effective cryoprotectant while DMSO₂ is ineffective. This anomaly was investigated by relating observations on the phase behavior of DMSO and DMSO₂ in aqueous solutions to the recovery of human lymphocytes frozen in the presence of these compounds. The lack of cryoprotection in the presence of DMSO₂ appears to be due to the precipitation of DMSO₂ from the solution at subzero temperatures. The observation of reduced cell recovery after freezing with increasing concentrations of DMSO₂ implies that cell damage is related to the amount of solid DMSO₂ present. Precipitation of DMSO₂ occurs both intra- and extracellularly, but it is argued that intracellular precipitation of DMSO₂ is the damaging phenomenon. Cryoprotective compounds are normally selected based on the criteria of low toxicity and permeability to the plasma membrane. An additional condition, solubility, must be included for interpretation of experimental data and for development of effective protocols for cryopreservation.