From uptake measurements of NO2 on solid amorphous carbon in the mTorr pressure range and at ambient temperature an accommodation coefficient for NO2 of (4.8¿0.6)¿10-2 has been measured. These experiments reveal a complex chemical mechanism. The only major product observed is NO whose yield corresponds to about 60% of the NO2 adsorbed on the surface. Thermal desorption of a carbon sample that has been exposed to NO2 yields CO and CO2 along with some NO3 and/or N2O5. H2O does not interact measurably at ambient temperature with the type of carbon used in the present study. ¿ American Geophysical Union 1993