The paper deals with the relationship between phonetics and phonology using examples from the vowel systems of four Roman languages – French, Spanish, Italian, and Romanian. Starting from the principles of the Prague Linguistic Cicrle and N. Trubetzkoy’s Phonology Theory, which built his theory of phonetics as a discipline studying the material of physical aspects of minimal linguistic units as speech elements and using the methodology of natural sciences, and of phonology as a discipline concerned with the distributive properties of a limited number of minimal linguistic units i.e. phonemes, elements belonging to language and which applies methodologies specific to linguistics, sociology and psychology in its research. A comparative analysis of the vowel systems of the four Roman languages revealed different approaches in classifying identical or very similar vocal realizations, but also similarities in their nature. The research showed that complex systems tend to be less stable and tend to reduce, while simpler ones are more stable. Certain allophonic realizations in one or more languages appear as full-fledged phonemes in the system of another language, whereas some other realizations, present in spontaneous speech but not in the phonological system of a specific language, emerge as phonemic units in another language.