Recent studies have shown that mathematical vocabulary is an important predictor for mathematical performance in kindergartners, primary school students, and secondary school students (e.g., King & Purpura, 2021; Peng & Lin, 2019; Ünal et al., 2021). However, a recent meta-analysis showed that mathematical vocabulary might be more important in some mathematical domains than in others (Peng et al., 2020). The current manuscript focusses on proportional reasoning, a mathematical subdomain containing a high amount of proportional vocabulary (a subtype of mathematical vocabulary), and consists of two studies. The first study describes the development of an instrument specially created to measure proportional vocabulary, as there was no good instrument available. The results provide indications that the scores on our new proportional vocabulary task are valid and reliable. This instrument was used in a second study to investigate whether second graders' proportional vocabulary is associated with (1) proportional reasoning abilities, and (2) performance on proportional and additive word problems. The results showed that after controlling for gender, age and class, proportional vocabulary uniquely contributed to proportional reasoning abilities and performance on both proportional and additive word problems.