Diamondoids are nanometer-sized, hydrogen terminated diamond-like molecules consisting of fused adamantane units. The thiolated diamondoid tetramantane-6-thiol shows negative electron affinity behavior, i.e. population of unoccupied states directly leads to spontaneous electron emission. We present time-resolved photoemission data from self-assembled monolayers of tetramantane-6-thiol in order to shed light on the emission process: A photon energy threshold for electron emission of 5.6–5.8 eV was observed, and the electron affinity was estimated to be −0.21 to −0.57 eV for Ag and Au substrates, respectively. Electrons are emitted after excitation in the metal substrate through the molecular orbitals within a few femtoseconds.