一氯甲烷又称甲基氯，无色、可燃、有毒气体，属有机卤化物。分子式是CH₃Cl，分子量是50.49。

• 沸点：-24 °C（760mmHg）
• 液体比重（D₂O）：0.95
• 溶解度：（水中，25 °C）：0.95
• 爆炸极限（空气中V﹪）：8.1-17.2
• 最高允许浓度：5 mg/m

Chloromethane, also called methyl chloride, Refrigerant-40, R-40 or HCC 40, is a chemical compound of the group of organic compounds called haloalkanes. It was once widely used as a refrigerant. It is a colorless extremely flammable gas with a mildly sweet odor, which is, however, detected at possibly toxic levels. Due to concerns about its toxicity, it is no longer present in consumer products. Chloromethane was first synthesized by the French chemists Jean-Baptiste Dumas and Eugene Peligot in 1835 by boiling a mixture of methanol, sulfuric acid, and sodium chloride. This method is similar to that used today.