Pronunciation is an essential aspect in the teaching of the English language, especially those aspects of pronunciation such as stress and vowel quality as they are crucial elements to ensure intelligibility in communication. The general objective of this study is to investigate whether the theoretical-practical instruction on pronunciation has a crucial impact on the vowel quality production of stressed and unstressed syllable in isolated words and in wider contexts, and therefore, in the improvement of intelligibility and of the oral production in general terms, in two groups of Spanish students of English (the control and the experimental group). More particularly, the study addresses the impact of formal instruction in pronunciation based on deduction in terms of rule formation from a cognitive perspective. The results indicate that the specific work implemented with the production of vowel quality in stressed and unstressed syllables have a significant impact on intelligibility.