The themes discussed in this study relate to how students reason about the information encoded in rate constants, which is important for developing a deep understanding of chemical kinetics at the molecular level. This study is part of a larger project centered more generally on students' understanding and use of mathematics in chemical kinetics. During semistructured interviews, students--36 general chemistry students, five physical chemistry students, and three chemical engineering students--worked through chemical kinetics problems and were asked to discuss equations used in chemical kinetics, yielding rich data that provided insight regarding students' understanding of rate constants. Analysis revealed that students often conflated ideas from chemical kinetics and equilibrium, such as rate constants and equilibrium constants. Furthermore, students demonstrated varying levels of sophistication regarding the distinction and relationship between rate and rate constants. Finally, students conveyed different ideas about the mathematical nature of the rate constant quantity. These findings suggest students need more support in order to develop a more sophisticated understanding of rate constants.