Using temperature programmed desorption (TPD), predosed oxygen TPD (POTPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron and X-ray photoelectron spectroscopy (AES and XPS), we have investigated the chemistry of chloroiodomethane (ClCH2I) dosed onto clean, D-covered and O-covered Pt(111). At or below 100 K, ClCH2I adsorbs molecularly on all these surfaces. While ClCH2I in physisorbed multilayers desorbs reversibly, a significant portion in the first monolayer dissociates during hearing. In the absence of D and O, dissociation begins with C-I bond cleavage at similar to 150 K. Once the C-I bond breaks, several competitive reactions take place below 260 K : (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, (2) Cl-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehydrogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapidly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, and (5) HCl and HZ formation and desorption above 200 K. : (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, (2) Cl-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehydrogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapidly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, and (5) HCl and HZ formation and desorption above 200 K. : (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, (2) Cl-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehydrogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapidly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, and (5) HCl and HZ formation and desorption above 200 K.