Commercially available speech synthesizers are now being used extensively in talking machines. As their numbers continue to grow, it is essential to establish standard evaluation tests as quality control checks in their manufacture. A contribution towards this goal is described in this paper. Listeners heard the output of two commercial synthesizers. By identifying the basic units of speech (phonemes) which are wrongly perceived and comparing these with known confusions arising in listening to real speech under difficult conditions, it was possible to identify the limitations of a given synthesizer.