This Event Related Potentials (ERP) study investigates auditory processing of sentences with so-called code-switches in Russian learners of German. It has often been argued that switching between two languages results in extra processing cost, although it is not completely clear yet what exactly causes these costs. ERP presents a good method to (start to) answer this question, since different ERP effects provide insight in the underlying processes that take place. We presented three groups of speakers (German first-language speakers, Russian speakers with intermediate and Russian speakers with very good knowledge of German as a second language) with German sentences that either ended "normally" i.e. with a word that fitted the meaning of the sentence, or with a semantically unexpected word, or with the Russian translation of the semantically normal German word. Comparing the two groups of Russian speakers allows for examining the influence of proficiency on the processing of code-switches. The results showed that the semantically unexpected word elicited an N400 in both the first language (L1) and the more proficient second language (L2) group, but not in the less proficient L2 group. Code-switches resulted in an N400-like pattern in all three groups, and also in a Late Positive Component (LPC), which was most pronounced in the less proficient L2 group. This positivity is, although somewhat later, quite similar to the well-known P300 effect that is found with stimuli with unexpected external properties.