We report an experimental observation of H+ and I- being formed following ultraviolet photolysis of CH2I2 in liquid water. The concentration of H+ and I- are about the same and suggest a reaction that produces a HI leaving group. A preliminary ab initio investigation is reported for the O-H insertion reactions of the isodiiodomethane (CH2I-I) carbenoid species with H2O and 2H(2)O and these are compared to the related reactions for the dichlorocarbene (:CCl2) species. The CH2I-I carbenoid reacts with water via an O-H insertion/HI elimination reaction mechanism. These reactions produce a HI leaving group whereas the dichlorocarbene O-H insertion reaction has no need for a leaving group. The differing structures and properties of the CH2I-I and the :CCl2 species can account for the differences in their O-H insertion reactions. The CH2I-I species O-H insertion/HI elimination reactions can be catalyzed by a second water molecule when two water molecules are available and this occurs in a way similar to that previously observed for the reaction of :CCl2 with two water molecules. These results suggest that isopolyhalomethane molecules (like CH2I-I and others) are noticeably reactive towards O-H bonds like those found in water and can likely account for the strong acid formation observed experimentally after ultraviolet photolysis of polyhalomethanes in water. Possible implications for the photochemistry of polyhalomethanes in the atmosphere and oceans are briefly discussed. (C) 2003 American Institute of Physics.