Scanning tunneling microscopy measurements on Ni(110) suggest that the segregation of hydrogen into the subsurface (beta(3)) state is mediated by CO. At 375 and 390 K exposure of the saturated added row Ni-H overlayer to 1 x 10(-8) Torr of CO(g) induces the dissolution of the Ni-H rows to yield Ni islands of monatomic step height. Structures associated with the adsorption of CO onto the clean surface are not observed, and the surface formed under these conditions is resistant toward hydrogen uptake upon further H-2(g) exposure. This reconstruction is activated; it does not occur at temperatures below 370 K at the same pressure of hydrogen. With heating, H-2 is evolved from the beta(3) desorption state from this surface; no other gases are observed to desorb. Cooling the surface to room temperature permits formation of the "normal" Ni-H overlayer upon further H-2(g) exposure. These findings suggest that at temperatures above 370 K CO induces a surface reconstruction with a fraction of the hydrogen from the Ni-H overlayer segregating into the subsurface, while the remainder evolves into the gas phase. Under a constant CO background pressure of 6 x 10(-8) Torr at 350 K a low-coverage c(4x2)-CO structure appears as unresolved [001]-oriented rows separated by four unit vectors along [110]. The mobility of CO along the [001] azimuth appears to result in a loss of surface registry of CO in that direction.