Single-atom substitution within a natural nucleobase—such as replacing oxygen by sulfur in uracil—can result in drastic changes in the relaxation dynamics after UV excitation. While the photodynamics of natural nucleobases like uracil are dominated by pathways along singlet excited states, the photodynamics of thiobases like 2-thiouracil populate the triplet manifold with near unity quantum yield. In the present study, a synergistic approach based on time-resolved photoelectron spectroscopy (TRPES), time-resolved absorption spectroscopy (TRAS), and ab initio computations has been particularly successful at unraveling the underlying photophysical principles and describing the dissimilarities between the natural and substituted nucleobases. Specifically, we find that varying the excitation wavelength leads to differences between gas-phase and condensed-phase experimental results. Systematic trends are observed in the intersystem crossing time constants with varying excitation wavelength, which can be readily interpreted in the context of ab initio calculations performed both in vacuum and including solvent effects. Thus, the combination of TRPES and TRAS experiments with high-level computational techniques allows us to characterize the topology of the potential energy surfaces defining the relaxation dynamics of 2-thiouracil in both gas and condensed phases, as well as investigate the accessibility of conical intersections and crossings, and potential energy barriers along the associated relaxation coordinates.