The aggregation of phenylacetylene in the gas phase was investigated by selectively recording the IR spectra of clusters consisting of up to six monomer units. Analysis of the IR spectra with the aid of B97-D3/aug-cc-pVDZ level calculations reveals the formation of an anti-parallel π-stacked structure of the dimer and a hitherto unknown assembly of clusters incorporating exclusively aromatic C–H⋯π interactions between various units of the trimer and higher clusters. The aggregation behaviour of phenylacetylene in the gas phase is fundamentally different from benzene, phenol and aniline vis-à-vis their crystal structures. The structures of the three known polymorphic crystals can be reconciled by the formation of supramolecular synthons with acetylenic C–H⋯π interactions, which is preferred over energetically favored aromatic C–H⋯π interactions. Furthermore, the small (phenylacetylene)_n [n = 3–6] clusters, the structures incorporating aromatic C–H⋯π interactions, can be envisaged as liquid-like aggregates which under varied conditions lead to the formation of multiple polymorphs during in situ cryo-crystallization.