This paper presents a two level lexical stress assignment model for out of vocabulary Slovenian words used in our text-to-speech system. First, each vowel (and consonant ’r’) is determined, whether it is stressed or unstressed, and a type of lexical stress is assigned for every stressed vowel (and consonant ’r’). We applied a machine-learning technique (decision trees or boosted decision trees). Then, some corrections are made on the word level, according the number of stressed vowels and the length of the word. For data sets we used the MULTEXT-East Slovene Lexicon, which was supplemented with lexical stress marks. The accuracy achieved by decision trees significantly outperforms all previous results. However, the sizes of the trees indicate that the accentuation in the Slovenian language is a very complex problem and a simple solution in the form of relatively simple rules is not possible.