Concatenative text-to-speech systems require an algorithm that allows prosodic modifications of the speech units during the concatenation process. Nowadays, sinusoidal modeling seems to be a promising technique to achieve very flexible algorithms that provide high quality synthetic speech. The main difficulty of these type of algorithms is the treatment of the phase information, since an inadequate processing of this information gives rise to reverberation and audible artefacts. In this contribution we discuss the application of a shape-invariant sinusoidal model [1] to a text-to-speech system based on concatenation of speech units.