Emotion differentiation (ED; the ability to distinguish discrete internal emotion states) may reflect or benefit from knowledge of linguistic labels. The present study uses natural language processing to examine how emotion vocabulary (EV; diversity of unique emotion terms within active vocabulary) relates to ED and depression in an adolescent sample. We tested two competing preregistered (https://osf.io/4j75w/) models regarding the EV-ED link. In the lexical facilitation hypothesis, we posited that larger EV may inform ED, perhaps resulting in larger EVs being associated with greater ED. In the emotional concision hypothesis, we theorized that ED may reflect narrower emotional experiences that are more succinctly labelled, which could result in larger EV being associated with lower ED. A community sample of adolescents (N = 241, ages 14-17, predominantly White) completed interviews, self-report measures, and ecological momentary assessments as part of a larger study conducted between 2014 and 2016. EV was derived using speech samples from transcribed recordings of life stress interviews. In line with the emotion concision hypothesis, EV and ED were inversely related for negative emotions. Moreover, larger negative EV and lower negative ED were each uniquely associated with depression, casting further doubt on whether diverse negative EVs within spontaneous language are fundamentally adaptive for emotional functioning. Replication in more diverse samples is needed to extend generalizability. (PsycInfo Database Record (c) 2024 APA, all rights reserved).