This paper aims at finding out the most common types of interlingual errors that Spanish students make when they are learning English as a foreign language. The subjects that have taken part in this study are a group of twenty eight first-year High School students. The interlingual errors obtained from the corpus have been divided into four types: transfer of structure, overextension of analogy, interlingual/intralingual and substitution. Each of these types has been analysed separately taking into account the wordclass where it occurs.