Diiodomethane, $CH_{2}I_{2}$, in a polar solvent undergoes a unique photoinduced reaction whereby $I_{2}^{−}$ and $I_{3}^{−}$ are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of $iso-CH_{2}I–I$, which is a major intermediate of the photodissociation, can account for the formation of $I_{2}^{−}$ and $I_{3}^{−}$, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, $CHI_{3}$, $C_{2}H_{4}I_{2}$, $C_{2}F_{4}I_{2}$, $I_{3}^{−}$, and $I_{2}$. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of $CH_{2}I_{2}$ in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that $I_{2}^{−}$ and $I_{3}^{−}$ are formed via heterolysis of $iso-CH_{2}I–I$ in the photodissociation of $CH_{2}I_{2}$ in methanol. In addition, we demonstrate that the high polarity of $iso-CH_{2}I–I$ is responsible for the unique photochemistry of $CH_{2}I_{2}$.