The ProSynth speech synthesis project aims to re-implement and extend the YorkTalk all-prosodic synthesis system in an open manner preserving its most ap-pealing theoretical aspects. A significant novel aspect of the architecture of ProSynth is the use of the extensible mark-up language (XML) as a computational formalism for the representation of hierarchical linguistic structures. The facilities provided by XML match closely the requirements to represent the phonological features of an utterance in a metrical prosodic structure, namely: nodes described by attribute-value pairs forming strict hierarchies. The XML formalism also leads to an elegant and efficient method for representing declarative phonological contexts under which phonetic interpretation is performed.