The difference between native speakers’ and non-native speakers’ naturalness judgements of synthetic speech is investigated. Similar/different judgements are analysed via a multidimensional scaling analysis and compared to Mean opinion scores. It is shown that although the two groups generally behave in a similar manner the variance of non-native speaker judgements is generally higher. While both groups of subject can clearly distinguish natural speech from the best synthetic examples, the groups’ responses to different artefacts present in the synthetic speech can vary.