This thesis has analysed the sound substitutions Carioca students of English make when speaking it. It was the researcher’s purpose to find out what type of mispronunciations occurred and to separate the errors caused by the orthographic representation of sounds from the ones coming from differences between the sound systems of American-English and Brazillian-Portuguese languages.By means of an error analysis the researcher has found out that the main cause of Cariocas’ sound substitutions were due to interference coming from the differences between the sound systems of the students’ native tongue and target language and the way sounds are distributed (71%) as compared to 30% of interference coming from orthographic representations. It was also interesting to note that the interference of orthography decreased in the free speech performance.