Guanosine monophosphate (GMP) in aqueous solutions has been studied with femtosecond broad-band transient absorption spectroscopy and by quantum-mechanical calculations. The sample was excited at 267 or 287 nm and probed between 270 and 1000 nm with 100 fs resolution, for various pH values between 2 and 7. At pH 2, when the guanine ring is ground-state protonated (GMPH(+)), we observe isosbestic behavior indicating state-to-state relaxation. The relaxation is biexponential, tau(1), = 0.4 ps, tau(2) = 2.2 ps, and followed by slower internal conversion with tau(3) = 167 ps. For nonprotonated GMP in the pH range 7-4, we find biexponential decay in the region 400-900 nm (tau(1), = 0.22 ps, tau(2) = 0.9 ps), whereas, between 270 and 400 nm, the behavior is triexponential with one growing, tau(1), = 0.25 ps, and two decaying, tau(2) = 1.0 PS, tau(3) = 2.5 ps, components. The excited-state evolution is interpreted with the help of quantum-chemical calculations, performed at the time-dependent PBEO level accounting for bulk solvent effects and specific solvation. The computed dynamics involves L-a and L-b bright excited states, whereas the n(0)pi* and pi sigma* dark excited states play a minor role. Independent of the pH, the photoinduced evolution involves ultrafast L-b-->L-a conversion (tau(ba) << 100 fs) and exhibits the presence of a wide planar plateau on L-a. For neutral GMP a barrierless path connects this region to a conical intersection (CI) with the ground state, giving an account of the ultrafast decay of this species. For protonated GMPH(+) the system evolves into a stable minimum L-a min characterized by out-of-plane displacement of NH and CH groups, which explains the longer (167 ps) fluorescence lifetime.