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Lexical ERPs (event-related potentials) obtained in an oddball paradigm were suggested to be an index of the formation of new word representations in the brain in the learning process: with increased exposure to new lexemes, the ERP amplitude grows, which is interpreted as a signature of a new memory-trace build-up and activation. Previous learning studies using this approach have, however, mostly used meaningless novel word forms; it therefore remains uncertain whether the increased amplitude simply reflects increased familiarity with the new stimulus or is indeed a reflection of a complete word representation. Here, we used the oddball paradigm to measure the mismatch negativity (MMN) responses to novel word forms before and after semantic training, during which they were associated with previously familiar words of either high or low frequency of occurrence. Following training, the amplitude of the MMN to novel words was enhanced. Furthermore, these changes were dependent on the frequency of the reference which novel items became associated with: namely, the MMN amplitude became greater and the latency shorter for the item which was assigned the high-frequency meaning. Even though the amount of training was the same for both types of items, the low-frequency stimulus did not achieve similar significant changes. Our results suggest that the new surface form becomes linked to the existing representation, which then automatically activates in full when the respective stimulus is present at the input. This finding indicates that the learning-related MMN dynamics, manifest as a response increase after learning, likely reflects the formation and activation of a complete lexicosemantic memory circuits for words.