Non-separable preference is defined as an individual's preference on one issue being conditional to the outcome of another issue. Studies in political behavior suggest three theories explaining the non-separable preferences: (1) High cognitive capacity with strong policy preferences, (2) motivated partisan independence, and (3) non-attitude. This article exploits a new rank order question design implemented right before the 2021 Taiwan referendum, in which two power outages before the voting encouraged the formation of non-separable preferences on two referendums choosing between environmental protection and power supply. Results of a pre-registered survey (n = 910) show that the majority of the self-reported non-separable voters are driven by non-attitude instead of policy or partisan concerns, even though these non-separable voters can alter the referendum results at the aggregate level. Non-separable voters have, on average, lower education, lower political knowledge, and are non-partisan. Its implications for forming and measuring non-separable preferences are finally discussed.