This paper investigates the thermal profiles that arise in oxygen VSA, which is a prominent example of a PSA bulk gas separation process. Experimentally, it is demonstrated that the severe axial thermal profile or "cold spot" that frequently characterizes oxygen VSA can only arise if there are multilayered adsorption beds or if there are readily adsorbed trace components (such as water) that create a de facto multilayered bed. A qualitative explanation is offered to explain how this cold spot is formed. This paper also details a technique for predicting the penetration of a water-loaded zone into an oxygen VSA adsorption bed based on the method of characteristics. The results of this technique compare well with experimental and numerically simulated results. Finally, this paper demonstrates that a water-loaded zone and an inert zone of activated alumina result in very similar cyclic steady-state thermal profiles, even though the transient behaviors are markedly different.