For people living with mental illness, support from social network members, including family, romantic partners, and friends, is critical but often inadequate. However, robust theoretical explanations for why it might it be difficult to support people living with mental illness are lacking. We assessed an appraisal-based model of uncertainty, hypothesizing that fear and anxiety and supportive communication efficacy mediate the association between mental illness uncertainty and support provision (i.e., quantity and quality). We also predicted that perceptions of others' communication about their mental illness (i.e., ambiguity and volume) would exacerbate feelings of fear and anxiety in relation to uncertainty. Analyses of data gathered from 300 individuals across the United States (Mage = 34.07, SD = 9.49; 50% female) supported our central prediction; uncertainty had a negative direct or indirect effect on all support types and overall support quality, partly through decreases in supportive communication efficacy. Counter to predictions, fear and anxiety positively predicted support quantity for all types. Findings of this study point to a difficult situation for social network members: although uncertainty seems to motivate support provision by activating fear and anxiety, this effect could be undermined for forms of support that require complex communication skills (e.g., emotional, esteem, overall quality) through corresponding decreases in supportive communication efficacy. We discuss the theoretical and practical implications of this research for social support and relationships within the context of mental illness.