Chemistry students often struggle in organic chemistry courses. In fact, these courses are viewed by some as “weed-out” classes. There are many fundamental concepts covered in general chemistry that contribute to students’ ability to succeed in organic chemistry. One of those fundamental topics, and the focus of this study, is the topic of acids and bases. Acid–base topics are featured in both general and organic chemistry courses, and the interests of this study lie not only within the realm of organic chemistry but also general chemistry. The purpose of this study was to determine both undergraduate general and organic chemistry students’ understandings of acid–base topics. The study design was guided by the theoretical framework of phenomenography, which is focused on identifying the various ways a group experiences a given phenomenon or event. In the case of this study, the phenomenon is the topic of acid–base chemistry. Specifically, I used semi-structured interviews and activities involving molecular structures in order to determine what knowledge or information students focused on and made use of when making decisions about the identity and behaviors of acids and bases. The end goal was to obtain a set of hierarchically organized categories that represent each participant group’s range of understandings of acids and bases. There were three main themes in terms of the knowledge and information that both participant groups used when making decisions about acidity and basicity: recognition, composition and structure, and behavior. In essence, participants appeared to be asking themselves certain questions when deciding if a molecule was an acid or a base: (a) “Do I know this molecule is an acid or a base?” (when relying on recognition of a molecule), (b) “Does this molecule look like an acid or a base?” (when focusing on the composition and structure of a molecule), and (c) “Does this molecule act like an acid or a base?” (when using behavior of a molecule to inform their decisions). When making decisions about the relative acidity of two different molecules, they appeared to be asking themselves one of an analogous set of questions: (a) “Do I know this molecule is more acidic or more basic than this other one?” (when relying on recognition of the molecules), (b) “Does this molecule look more acidic or more basic than this other one?” (when focusing on the composition and structure of the molecules), and (c) or “Does this molecule act more like an acid or more like a base than this other one?” (when using behavior of a molecule to inform their decisions). The data revealed that participants associated particular aspects of structure, composition, and behavior with acidity and basicity, and that many of these aspects were related to the definitions of “acid” and “base” provided by the three models of acidity (Arrhenius, Brønsted-Lowry, and Lewis). For example, participants often brought up the ability of a molecule to donate hydrogen when deciding a molecule was an acid (the Brønsted-Lowry definition of an acid) and the presence of lone pair electrons when determining that a molecule was a base (a Lewis base is defined as a species that donates an electron pair). Overall, although some participants were able to use underlying chemical principles to explain their identification of acids and bases or to choose the more acidic (basic) of a pair of molecules, most participants—at both the general chemistry and organic chemistry levels—relied on simple, surface-level features of molecules when discussing the acidity or basicity of a molecule.