Teachers' emotional exhaustion is related to various detrimental outcomes, such as work absenteeism, intention to quit, impaired instructional quality, and lower student motivation. Since emotional exhaustion becomes evident as early as teacher training at university, the question is whether it would be possible to identify an individual disposition to this burnout symptom in student teachers. Accordingly, it is important to gain a deeper understanding of the genesis, stability, and trajectories of change of emotional exhaustion. In the present study, we have disentangled different components of longitudinal stability in emotional exhaustion by applying STable, AutoRegressive Trait, and State models to two longitudinal samples of German student teachers (Sample 1: N = 4,510; Sample 2: N = 2,034). Over the timespan of up to 6 years, we found that completely stable sources (Sample 1: 35%; Sample 2: 37%) and slowly changing factors (Sample 1: 46%; Sample 2: 36%) in student teachers' emotional exhaustion accounted for much of the total variance in interindividual differences; occasion-specific factors and measurement error accounted for the remaining variation (Sample 1: 23%; Sample 2: 30%). This variance composition applied equally to male and female students and indicated that student teachers' emotional exhaustion comprises a stable component but is predominantly characterized by malleable parts due to systematically changing and occasion-specific sources of variance. Our findings about the stability of emotional exhaustion contribute to the trait-state debate of psychological characteristics and point out that intervention measures should be employed at an early stage to mitigate the negative consequences of burnout in subsequent career steps.