Titration experiments using ten reactive probe gases interacting with a Powdered Activated Carbon (PAC) sample have been performed in a two-chamber Knudsen flow reactor equipped with molecular-beam modulated mass spectrometric detection. The goal of these uptake and desorption experiments performed at ambient temperature was to establish a map of reactive sites at the PAC interface of 1580 m(2)/g BET surface area. The extent of the net uptake of the probe gases in molecule cm(-2) was in the order O-3 >> NO2 > TFA (trifluoroacetic acid, CF3COOH) >HA (hydroxylamine, NH2OH) > C4H4S (thiophene) > TMA (trimethylamine) > HCl > H2O > H2S > SO2. PAC may, therefore, be characterised as a reduced and weakly basic type of amorphous carbon. The sum of individual coverages amounts to 62.5% and corresponds to a highly decorated PAC interface. Six out of ten probe gases (H2O, NO2, HCl, TMA, TFA and thiophene (C4H4S)) spontaneously desorb to variable extents at ambient temperature under flow reactor conditions from a previously saturated PAC interface. The adsorption/desorption rate coefficients were fitted to the experimental desorption rates using an analytical solution. For HCl, NO2 and H2O a rather large discrepancy of a factor of 7.3, 12.7 and 71.4, respectively, between the initial uptake rate constant k(0) on a virgin non-polar PAC surface and a gas-saturated PAC surface upon readsorption occurring in spontaneous desorption experiments (k(a)) has been detected.