The intermolecular interactions responsible for the microsolvation of the highly flexible trimethylene oxide (TMO) and trimethylene sulfide (TMS) rings with one and two water (w) molecules were investigated using rotational spectroscopy (8–22 GHz) and quantum chemical calculations. The observed patterns of transitions are consistent with the most stable geometries of the TMO–w, TMO–(w)2 and TMS–w complexes at the B2PLYP-D3(BJ)/aug-cc-pVTZ level and were confirmed using spectra of the 18O isotopologue. Due to its effectively planar backbone, TMO offers one unique binding site for solvation while water can bind to the puckered TMS ring in either an axial or equatorial site of the heteroatom. In all clusters, the first water molecule binds in the σv symmetry plane of the ring monomer and serves as a hydrogen bond donor to the heteroatom. The second water molecule is predicted to form a cooperative hydrogen bonding network between the three moieties. Secondary C–H…O interactions are a key stabilizing influence in the trimers and also drive the preferred binding site in the TMS clusters with the axial binding site preferred in TMS–w and the equatorial form calculated to be more stable in the dihydrate. Using an energy partition scheme from symmetry-adapted perturbation theory for the O, S and Se containing mono- and dihydrates, the intermolecular interactions are revealed to be mainly electrostatic but the dispersive character of the contacts is enhanced with increasing size of the ring’s heteroatom due to the key role of longer-range secondary interactions.