Quantitative aspects related to atmospheric-pressure Penning ionization (APPeI) were examined by analyzing C6D6 in C6H6 using C6H6 as an internal standard. A linear relation was found in log῎log plots betweenand the amounts of C6D6 detected were in the range from 1.6῎10 ῍10 g to 1.6῎10 ῍4 g for C6D6 spiked in 0.4 mL C6H6, with a correlation coe$cient of 0.995. The limit of detection for C6D6 was approximately tens of pg. In He and Ar APPeI, a major reagent for Penning ionization would be the rare gas (Rg) excimers Rg2ῑ.