Methyl chloroform (1,1,1-trichloroethane, CH<sub>3</sub>CCl<sub>3</sub>) was found to decompose heterogeneously on seven types of standard clay minerals (23 materials) in dry air at 313 K in the laboratory. All reactions proceeded through the elimination of HCl; CH<sub>3</sub>CCl<sub>3</sub> was converted quantitatively to CH<sub>2</sub>=CCl<sub>2</sub>. The activities of the clay minerals were compared via their pseudo-first-order reaction rate constants (<i>k</i><sub>1</sub>). A positive correlation was observed between the <i>k</i><sub>1</sub> value and the specific surface area (<i>S</i>) of clay minerals, where the <i>S</i> value was determined by means of the general Brunauer-Emmett-Teller (BET) equation. The <i>k</i><sub>1</sub> value was anti-correlated with the value of <i>n</i>, which was a parameter of the general BET equation and related to the average pore size of the clay minerals, and correlated with the water content that can be removed easily from the clay minerals. The reaction required no special pretreatment of clay minerals, such as heating at high temperatures; hence, the reaction can be expected to occur in the environment. Photoillumination by wavelengths present in the troposphere did not accelerate the decomposition of CH<sub>3</sub>CCl<sub>3</sub>, but it induced heterogeneous photodecomposition of CH<sub>2</sub>=CCl<sub>2</sub>. The temperature dependence of <i>k</i><sub>1</sub>, the adsorption equilibrium coefficient of CH<sub>3</sub>CCl<sub>3</sub> and CH<sub>2</sub>=CCl<sub>2</sub>, and the surface reaction rate constant of CH<sub>3</sub>CCl<sub>3</sub> were determined for an illite sample. The <i>k</i><sub>1</sub> value increased with increasing temperature. The amount of CH<sub>3</sub>CCl<sub>3</sub> adsorbed on the illite during the reaction was proportional to the partial pressure of CH<sub>3</sub>CCl<sub>3</sub>. The reaction was sensitive to relative humidity and the <i>k</i><sub>1</sub> value decreased with increasing relative humidity. However, the reaction was found to proceed at a relative humidity of 22% at 313 K, although the <i>k</i><sub>1</sub> value was about one-twentieth of the value in non-humidified air. The conditions required for the reaction may be present in major desert regions of the world. A simple estimation indicates that the possible heterogeneous decomposition of CH<sub>3</sub>CCl<sub>3</sub> on the ground surface in arid regions is worth taking into consideration when inferring the tropospheric lifetime of CH<sub>3</sub>CCl<sub>3</sub> and global OH concentration from the global budget concentration of CH<sub>3</sub>CCl<sub>3</sub>.