Volunteers have played an important role by supporting essential services that have been overwhelmed during the most critical moments of the SARS-CoV-2 pandemic. Hence, nonprofit organizations may be interested in preventing negative consequences of these volunteers' exposure to potentially traumatic events. The aim of this cross-sectional study was twofold. First, to examine to what extent self-compassion and self-determination would contribute to differentiating between volunteers with different levels of compassion fatigue, compassion satisfaction, and post-traumatic growth. Second, to identify the best predictors of the most extreme levels of each outcome. Participants were 211 Spanish Red Cross volunteers (60.7% women), who completed a survey. They were separately classified into three groups (low, medium, and high) according to the 33rd and 66th percentile scores on each outcome (compassion fatigue, compassion satisfaction, and post-traumatic growth). Univariate analyses of variance and post-hoc comparisons revealed that self-compassion and self-determination contributed differently to distinguishing between levels of each outcome. Volunteers lowest in compassion fatigue stood out for showing fewer non-compassionate strategies and more mindfulness than the other groups. Moreover, those higher in satisfaction compassion also showed lower use of unhealthy strategies and higher scores in all other predictive variables. Volunteers highest in post-traumatic growth showed higher self-kindness and satisfaction of all psychological needs. Binary logistic regressions allowed for the identification of predictors of belonging to the most extreme groups. The protective factors may be useful to guide volunteers' self-care and help them thrive in the face of critical service demands.