The geometry, conformational behavior, and magnetic properties of 2-(2-imidazolyl-)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, an important spin carrier in molecular magnetic materials, have been studied by a combined experimental and theoretical approach. From the experimental point of view, the available structural data have been completed by new electronic spin resonance spectra in different solvents. From the computational point of view, we have used a hybrid Hartree-Fock/density functional method which provides very reliable structural data. Next, the properties computed at this level have been corrected with reference to refined post Hartree-Fock computations for a smaller model system. Solvent effects have been taken into account by the polarizable continuum model, and crystal field effects have been mimicked by a suitable model cluster. Our computations show that the molecule has a planar structure in the gas phase and in solution, even if the rotational barrier significantly decreases with the polarity of the solvent. In contrast, strong intermolecular hydrogen bonds favor a nonplanar structure in the solid state. As a consequence, a significant modification of the molecular properties is observed going from vacuo to different condensed phases. All of these results are in good agreement with experiments and point out the interpretative power of our integrated computational tool.