An ocean general circulation model (OGCM) is used to simulate the direct injection of CO₂ near Tokyo. Our results confirm that direct injection can sequester large amounts of CO₂ from the atmosphere when disposal is made at sufficient depth but show that the calculated efficiency is sensitive to the choice of physical model. Moreover, we show, in an OGCM and under a reasonable set of economic assumptions, that sequestration effectiveness is quite high for even shallow injections. However, the severe acidification that accompanies injection and the impossibility of effectively monitoring injected plumes argue against the large-scale viability of this technology. Our coarse-grid models show that injection at the rate of 0.1 Pg-C/yr lowers pH near the site of injection by as much as 0.7-1.0 pH-unit. We also show that, after several hundred years, one would effectively need to survey the entire ocean in order to accurately verify the inventory of injected carbon. These results suggest that while retention may be sufficient to justify disposal costs, other practical problems will limit or at best delay widespread deployment of this technology.