The optically excited electronic states of hydrogen-bonded homo- and heterodimers of guanosine (G) and deoxycytidine (C) were investigated by femtosecond fluorescence up-conversion spectroscopy. The base pairs were prepared in CHCl3 solution by employing tert-butyldimethylsilyl (TBDMS) groups at the OH positions of the ribose (G) or deoxyribose (C) moieties to enhance the solubilities of the nucleosides in organic solvents. The H-bonded complexes that were obtained were characterized by FTIR spectroscopy. Fluorescence lifetime measurements were performed following electronic excitation at a series of UV wavelengths from lambda(pump) = 294 rim, close to the electronic origins of the bases, to lambda(pump) = 262 nm, where significant excess vibronic energy is deposited in the molecules, at nucleoside concentrations of c(0) = 0.1 and 1.0 mM. The experimental results revealed the existence of all ultrafast deactivation pathway for the optically prepared electronically excited state(s) of the G center dot C Watson-Crick base pair, which was found to have a lifetime of tau(GC) = 0.30(3) ps (with 2 sigma error limits) irrespective of the Pump wavelength. A similar short decay time, tau(GG) = 0.32(2) ps, was observed for the respective excited G center dot G homodimer. In contrast, the excited G monomer displayed a significantly longer-lived and wavelength-dependent deactivation, requiring three time constants, between 0.43(6) ps <= tau(G,1) <= 1.2(1) ps, 4.2(8) ps <= tau(G,2) <= 8(1) ps, and tau(G,3) = 195(32) ps. Self-complexation of C, on the other hand, led to a longer-lived excited state with a lifetime estimated between 1 ps <= tau(CC) <= 10 ps, compared to the dominant initial subpicosecond decay time or the C monomer of tau(C,1) = 0.80(4) ps.