A successful thermal storage system, useful in the 115 K to 120 K temperature range, has been designed and demonstrated in a typical IR sensor satellite environment. Because of its high heat of fusion, chemical stability, compatibility with aluminum, and its low room temperature vapor pressure, 2-methylpentane (2-MP) appeared to be the best phase change material (PCM ) in this temperature range. The pure material, however, exhibited severe supercooling that depresses the temperature at which freezing occurs by as much as 30 K below its normal 119.3 K freeze point, thereby making it totally unsuitable. Other hydrocarbons tested exhibited the same behavior. An extensive investigation found that supercooling was significantly reduced with the addition of 3% acetone which acts as a nucleation catalyst that aids crystallization. Repeated freeze/thaw cycling during orbital simulation testing in a vacuum environment showed supercooling to be essentially eliminated resulting in stable, predictable operation. The work described here can be applied to thermal storage systems operating at lower temperatures where supercooling is likely to occur.