We have recently found that in-diffusion of hydrogen into n-type Si crystals containing oxygen and carbon impurities can result in the formation of powerful recombination centers (M. Vaqueiro-Contreras et al., to appear in PSS RRL). Here, we describe a combination of first-principles calculations and electrical measurements to investigate the composition, structure, electrical activity and recombination mechanism of a carbon-oxygen-hydrogen complex (COH) in Si. We found a defect comprising a carbon-oxygen complex connected to an H atom whose location depends on the charge state of the complex, and showing a calculated acceptor level at Ev+0.3eV, a few meV away from the observations. Bistable carbon-oxygen-hydrogen complex in silicon. Carbon, oxygen, hydrogen, and silicon atoms are shown in gray, red, black, and white, respectively