The extant theory of ethnic defection rests upon the importance of ethnic identity shift and loyalism towards the regime, which were thus far presented as key explanations of side-switching in ethnic conflicts. This article challenges the validity of these claims and proposes an alternative theoretical argument. This study argues that individuals mobilise against their co-ethnics on the side of government that explicitly challenges and opposes ethno-nationalist aspirations of their own ethnic group due to perceived obligation of ethnic responsibility to protect their ethnic values. Ethnic defection is likely to occur when and if the rebels are suspected by their co-ethnics of violating or disregarding sociocultural, ideological or religious values of their ethnic group. Third-party pro-rebel intervention is likely to further aggravate or even trigger ethnic defection. This argument is examined empirically on the case of pro-government mobilisation during East Ukraine conflict in 2014-15. Micro-level interview data from Ukraine, demonstrates that Ukrainian Russianspeakers mobilised for the government side driven by the strong sense of ethnic responsibility, engendered in the perception that separatists misrepresent ethnic values of Ukraine's Russian-speakers. and religious principles and ethics peculiar to a particular ethnic group. Ethnic responsibility is engendered in an intrinsic obligation to protect ethnic values from perceived intra-group threats, which may include disregard for ethnic traditions, organisational structures, religious beliefs or collaboration with external actors. The logic of ethnic responsibility rests upon an assumption that ethnic rebels always remain at risk of alienating their co-ethnics, who might perceive their actions as contradictory to ethnic values, and as a result collaborate with the incumbent. Differences in interpretation of ethnic valuescultural, ideological, religious and customary perceptionspitch co-ethnics against each other, justifying and facilitating ethnic defection. The emergence of ethnic responsibility is often triggered by external pro-rebel ideological or military intervention. It is also embedded in inherent material motivations associated with mobilisation for a state actor. This theoretical claim is tested empirically on the case study of ethnic defection amongst Russian-speaking pro-government volunteers in East Ukraine's conflict. The empirical data derives from 86 in-depth interviews with former and active pro-government combatants from ethnic Russian and Russian-speaking communities. Findings demonstrate that Russianspeakers from eastern, southern and central Ukraine, who mobilised en mass into progovernment armed forces, were heavily influenced by the collective perception that pro-Russian separatists are misrepresenting ethnic Russian values. As members of their ethnic Russian community, recruits associated pro-Russian uprising in the East Ukraine with external ideological and military intervention. This article proceeds as follows. Thе study begins wit...