This study analyzed the electrophysiological correlates of language switching in second language learners. Participants were native Spanish speakers classified in two groups according to English proficiency (high and low). Event-related potentials (ERPs) were recorded while they read English sentences, half of which contained an adjective in Spanish in the middle of the sentence. The ERP results show the time-course of language switch processing for both groups: an initial detection of the switch driven by language-specific orthography (left-occipital N250) followed by costs at the level of the lexico-semantic system (N400), and finally a late updating or reanalysis process (LPC). In the high proficiency group, effects in the N400 time window extended to left anterior electrodes and were followed by larger LPC amplitudes at posterior sites. These differences suggest that proficiency modulates the different processes triggered by language switches.