The organic compound 1,1,1-trichloroethane, also known as methyl chloroform, is a chloroalkane. This colourless, sweet-smelling liquid was once produced industrially in large quantities for use as a solvent. It is regulated by the as an ozone-depleting substance and its use is being rapidly phased out.

1,1,1-Trichloroethane was first reported by Henri Victor Regnault in 1840. Industrially, it is usually produced in a two-step process from vinyl chloride. In the first step, vinyl chloride reacts with hydrogen chloride at 20-50 °C to produce 1,1-dichloroethane:

This reaction is catalyzed by a variety of Lewis acids, mainly aluminium chloride, iron(III) chloride, or zinc chloride. The 1,1-dichloroethane is then converted to 1,1,1-trichloroethane by reaction with chlorine under ultraviolet irradiation:

This reaction proceeds at 80-90% yield, and the hydrogen chloride byproduct can be recycled to the first step in the process. The major side-product is the related compound 1,1,2-trichloroethane, from which the 1,1,1-trichloroethane can be separated by distillation.

A somewhat smaller amount of 1,1,1-trichloroethane is produced from the reaction of vinylidene chloride and hydrogen chloride in the presence of an iron(III) chloride catalyst:

1,1,1-Trichloroethane is marketed with stabilizers since it is unstable with respect to dehydrochlorination and attacks some metals. Stabilizers comprise up to 8% of the formulation, including acid scavengers (epoxides, amines) and complexants. The Montreal Protocol targeted 1,1,1-trichloroethane as one of those compounds responsible for ozone depletion and banned its use beginning in 1996. Since then, its manufacture and use has been phased out throughout most of the world.

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