Previous research on mathematical problem solving has shown that people tend to select mathematical operations that are structurally aligned with the semantic relations between objects in a problem. In particular, taxonomically related objects (tulips and roses) elicit addition, whereas thematically related objects (tulips and vases) elicit division. This could be a strategic, conscious process, or it could be a more automatic tendency. Psycholinguistic research using event related potentials has established reliable differential brain responses to semantically anomalous words in a sentence - the N400 effect. If semantic alignment processes are a natural part of reading mathematical word problems, it is conceivable that misaligned mathematical expressions would elicit a similar effect. We compared ERPs elicited by semantically aligned (4 tulips + 2 roses) and semantically misaligned (4 tulips + 2 vases) math problems, to determine whether N400 amplitude can be used as an electrophysiological marker of misalignment.