The present study investigates the boundaries of speech and song from an acoustic-perceptual perspective. Using the speech-to-song illusion as a method, we tested rhythmic and tonal hypotheses in order to find out whether acoustic characteristics can cue the perceptual classification of a sentence by German listeners as sung or spoken. First, our results show that, despite individual differences, the speech-to-song illusion is a robust perceptual phenomenon comparable to those known in visual perception. Second, the experiment revealed that acoustic parameters – especially tonal structure – facilitate the perceptual shift from speech to song pointing to an acoustically guided decoding strategy for speech- vs. song-like signals.