The interaction of gold nanoparticles (AuNPs) and typical tetracyanoquinoidal compounds such as bis(dicyanomethylene)-bithiophene and tetracyanoquinodimethane (TCNQ) has been investigated. AuNPs in toluene solution reduce the tetracyano compounds to the radical anion, as shown by UV-vis spectroscopy. The reaction, promoted by the bromide anion used as a stabilizer for AuNPs, involves in the case of TCNQ the total amount of Au(0) in the nanoparticles. A spectrophotometric method for the evaluation of the Au(0) content of capped AuNPs in organic solution has been established and successfully applied to the analysis of dodecanethiol-capped AuNPs.