There has recently been a revival of interest in ‘naturalizing’ ethics. A naturalization seeks to vindicate ethical realism — the idea that ethical judgments can be true reflections of a moral reality — without violating the naturalist constraint that science sets the limits of ontology. The recent revival has been prompted by examples of successful scientific reduction (e.g. temperature, water), and by the emergence of new, nonreductive naturalist strategies (e.g. for biological and mental properties). In this paper, I argue against such naturalist approaches to ethics. My argument builds on the traditional one offered by G.E. Moore, namely that a naturalization would fail to respect an existing difference between the meanings of moral and naturalistic terms. I defend this line of argument against the common claim that it cannot block ‘synthetic’ property identities, ones grounded not in meaning equivalences but in empirical discoveries (as in the cases of temperature and water). I then go on to show that the Moorean argument can make trouble even for recent revisionist and nonreductive naturalist approaches.