This paper presents a perceptual evaluation of a text to speech (TTS) synthesizer in Greek with respect to acoustic registration of enclitic stress and related naturalness and intelligibility. Based on acoustical measurements and observations of naturally recorded utterances, the corresponding output of a commercially available formant-based speech synthesizer was altered and the results were subjected to perceptual evaluation. Pitch curve, intensity, and duration of the syllable bearing enclitic stress, were acoustically manipulated, while a phonetically identical phrase contrasting only in stress served as control stimulus. Ten listeners judged the perceived naturalness and preference (in pairs) and the stress pattern of each variant of a base phrase. It was found that intensity modification adversely affected perceived naturalness while increasing perceived stress prominence. Duration modification had no appreciable effect. Pitch curve modification tended to produce an improvement in perceived naturalness and preference but the results failed to achieve statistical significance. The results indicated that the current prosodic module of the speech synthesizer reflects a good balance between prominence of stress assignment, intelligibility, and naturalness.