Abstract: Emotion differentiation refers to the specificity with which people describe their emotional experiences. Low emotion differentiation has been linked to affective disorders but understudied in posttraumatic stress disorder (PTSD), despite a wide range of symptoms representing affective dysregulation (e.g., avoidance of trauma-related emotions). Additionally, although emotion differentiation is conceptualized as a skill that may improve with mental health treatment, it has been primarily studied as a trait rather than momentary construct, limiting opportunities to understand contextual factors. Using multilevel modeling, we examined predictors of momentary negative emotion differentiation (NED) and positive emotion differentiation (PED) in 63 US veterans in residential PTSD treatment completing ecological momentary assessment (EMA) of emotions five times a day for an average of 30 days. Predictors were initial PTSD severity (via clinical interview), time in treatment (day post-admission), and the presence of a service dog (a unique component of the treatment program). Covariates were mean negative affect or positive affect. After controlling for mean affect, PTSD severity was associated with both lower momentary NED and PED, highlighting these as potential treatment targets. Time in treatment did not emerge consistently as a predictor which may have been due to EMA occurring during the second half of treatment. The presence of a service dog was associated with higher momentary PED, which may parallel past findings on people experiencing higher momentary PED in more (versus less) familiar situations. Future research is needed to elucidate the time course and types of PTSD interventions in producing potential benefits in emotion differentiation.