Russellian monism – an influential doctrine proposed by Russell (1927/1992) – is roughly the view that the natural sciences can only ever tell us about the causal, dispositional, and structural properties of physical entities and not about their categorical properties, and, moreover, that our qualia are constituted by categorical properties. Recently, Stoljar (2001a, 2001b), Strawson (2008), Montero (2010, 2015), Alter and Nagasawa (2012), and Chalmers (2015) have attempted to develop this doctrine into a version of physicalism. Russellian monism faces the so-called combination problem, according to which it is difficult to see how categorical properties could collectively constitute qualia. In this paper, I suggest that there is an insufficiently discussed aspect of the combination problem which I call the difference-maker problem. Taking the difference-maker problem into account, I argue that the combination problem – whether or not it can be solved – results in a dilemma for the project of developing Russellian physicalism. That is, Russellian monism is either physicalistically unacceptable or it is implausible; hence, Russellian monism and physicalism are incompatible.