The objective of this investigation was to analyze the academic essays of fourth-year pre-service teachers of English of four Chilean universities. A linguistic taxonomy was elaborated in order to categorize the types of errors found under the grammatical and lexical dimensions, considering different studies on language error analysis. Errors were counted and classified by three judges, after which statistical analyses were applied using the Krippendorff's Alpha coefficient to determine whether the level of agreement among judges validated the final results. Representative examples taken from the essays for each type of error were analyzed according to their possible sources of error, and assumptions were made to determine their likely causes. Results showed that the grammatical dimension presented more errors than the lexical one.