The aim of the paper is to give a formal compositional semantics for Spiking Neural P systems (SNP systems) by following the Structural Operational Semantics (SOS) approach. A process algebra is introduced whose terms represent SNP systems. The algebra is equipped with a semantics, given as a labelled transition system. This semantics allows notions of behavioural equivalences over SNP systems to be studied. Some known equivalences are considered and their definition based on the given semantics is provided. Such equivalences are proved to be congruences.