The ground- and lowest excited-state properties of piperidine vapor are explored with respect to understanding its absorption and fluorescence properties. A ground-state intrinsic reaction coordinate (IRC) calculation was used to model the conformational potential energy surface connecting the equatorial and axial conformers. At the MP2/6-311++G** level of theory, the equatorial conformer is more stable by 310 cm(-1) than the axial conformer, and the inversion barrier height is 2033 cm(-1). Two transitions in the UV, with origins of 38 707 and 44 070 cm(-1) are assigned. The S-1 <-- S-0 transition (f(obs) 3.2 X 10(-3)) is Rydberg in nature, with considerable involvement of all the ring heavy atoms. A vibrational analysis of this transition shows a main progression in 640 cm(-1), which is assigned as the N-H out-of-plane bending motion. The CIS-calculated equilibrium geometry of the S-1 state indicates considerable distortion of the N atom relative to the C-alpha atoms. The one-dimensional absorption spectrum is modeled on the basis of the ground-state IRC and the corresponding vertical CAS(2,2)/MP2 surface. The IRC dependence of the transition moment was taken into account. The radiative rate constant, k(r), is estimated be ca. 1.7 x 10(6) s(-1). A weak fluorescence with (v) over tilde (max) = 33 500 cm(-1) is observed with a quantum efficiency, q(f), of ca. 1.3 x 10(-4). The lifetime of the S-1 state is estimated to be ca. 80 ps on the basis of q(f)/k(r). Deuterium substitution (d1 and d11) results in slight increases in q(f).