The primary photoreactivity of the excited states of trans-urocanic acid (t-UA) is investigated by ultrafast transient-absorption spectroscopy. Fundamentally different photophysics were observed when t-UA is excited at 266 nm, near the peak of the absorption spectrum, and 306 nm, in the red tail of the absorption spectrum. The data support the conclusion that the wavelength-dependent photophysics of t-UA is due to the presence of two different closely spaced electronic states. Excitation at 266 nm populates a pi pi* state that is localized on the imidazole ring. The transient data following photoexcitation of t-UA at 266 nm in both a pH 5.6 and a pH 7.2 solution are similar, even though the protonation state of the tertiary nitrogen on the imidazole ring is different at these two pH values. The data therefore support that the photophysics at pH 5.6 and pH 7.2 must involve a common excited state. Steady-state excitation spectra suggest that a proton transfer process from t-UA to the solvent occurs following the excitation at 266 nn at pH 5.6, which generates an electronically excited singlet state of the deprotonated molecule. This state is directly accessed by the 266 nm excitation of t-UA at pH 7.2. The population in this singlet state decays by intersystem crossing with a rate constant of 1.4 x 10(11) s(-1). Isomerization is not believed to occur from this triplet state. Excitation of t-UA at 306 nm populates an entirely different state, which leads to isomerization. From the observed ground state repopulation dynamics, the minimum rate for the excited state isomerization is 1.2 x 10(10) s(-1).