We investigated if incongruent solutions of simple multiplication problems would elicit similar event-related brain potentials as inappropriate words in sentences. In Experiment I, 12 subjects verified the appropriateness of solutions of multiplication problems or of final words in short sentences. Both incongruent solutions and incongruent words evoked a phasic negative shift between 300 and 500 ms having a similar topography. In Experiment II, we tested with another sample of 13 subjects if the amplitude of this arithmetic N400 effect was affected differently by different stimulus onset asynchronies (SOA = 200 and 500 ms) and by errors that were either table-related or table-unrelated to the preceding operands. Again, incorrect solutions elicited an arithmetic N400 effect whose amplitude depended on both the relatedness of the solution and the SOA. The ascending part of the N400 effect was always larger for unrelated than for related errors independently of the SOA, whereas the maximum of the N400 effect was larger for unrelated errors in case of a long SOA only. This pattern of effects was similar to that observed with semantic material varying lexical associations. These results suggest that arithmetic incongruencies are handled by the system in a manner functionally similar to that of semantic incongruencies.