In our paper we analyse the processes of diphtongization of the bivocalic sequences in Slovak language. The research was determined by instability in the usage and by various solutions in the dictionaries. The fuzzy character of the boundaries between diphtongs and vocalic phonemes is partly natural as it reflects the adaptation processes of foreign units. During this process the bivocalic sequences lose the character of vocalic phonemes and obtain the properties of diphtongs. We distinguish three degrees of this process which is determined by intern-lingual and extra-lingual factors.