Compared with other industrial processes, carbon capture and storage (CCS) will have an unusual impact on atmospheric composition by reducing the CO2 released from fossil-fuel combustion plants, but not reducing the associated O-2 loss. CO2 that leaks into the air from below-ground CCS sites will also be unusual in lacking the O-2 deficit normally associated with typical land CO2 sources, such as from combustion or ecosystem exchanges. CCS may also produce distinct isotopic changes in atmospheric CO2. Using simple models and calculations, we estimate the impact of CCS or leakage on regional atmospheric composition. We also estimate the possible impact on global atmospheric composition, assuming that the technology is widely adopted. Because of its unique signature, CCS may be especially amenable to monitoring, both regionally and globally, using atmospheric observing systems. Measurements of the O-2/N-2 ratio and the CO2 concentration in the proximity of a CCS site may allow detection of point leaks of the order of 1000 ton CO2 yr(-1) from a CCS reservoir up to 1km from the source. Measurements of O-2/N-2 and CO2 in background air from a global network may allow quantification of global and hemispheric capture rates from CCS to the order of +/- 0.4 PgCyr(-1).