People's engagement in altruistic behaviors depends on the relative importance given to values of humanistic altruism (HA) and biospheric altruism (BA). Specifically, while HA is considered the value base for prosocial behavior, BA is considered the value base of pro-environmental behavior. Despite the clear conceptual distinction, the two values often similarly correlate with outcome variables such as attitudes or choices and lead to ambiguous findings on the common versus unique impact of HA and BA on prosocial and pro-environmental behaviors. Here, we propose that the two types of altruism result in unique behavioral outcomes when they compete with each other, i.e., when people are forced to prioritize one value over the other. In two studies (N_total = 1163), we provided evidence for the assumed operational distinction between HA and BA. Moreover, we tested the self-activation hypothesis, the assumption that value centrality moderates the relationship between value activation and value expressive behavior. Results revealed that the experimental activation of HA and BA led to more value congruent behavior in people with high value centrality, but to reactance effects in people with low value centrality. Overall, this article offers new insight for the development of comprehensive theories of altruistic behavior.