The microsolvated complexes of formanilide, generated in a supersonic expansion, have been observed by Fourier transform microwave spectroscopy. Three 1 : 1 and one 1 : 2 formanilide-water adducts have been observed and their structures characterized by the measurement of isotopologue rotational spectra. In one of the monohydrated complexes and in the dihydrated complex, formanilide adopts a cis -configuration. In these species water closes sequential cycles with the cis amino and carbonyl groups through a network of N-H O and O-H O hydrogen bonds. Furthermore, in these complexes cis -formanilide has almost the same non planar configuration observed in the monomer. In the two monohydrated complexes detected with trans -formanilide, a planar skeleton is detected with water interacting solely with either the amino (N-H··O bond) or the carbonyl group (O-H O&z.dbd;C bond). The observed tunnelling splittings show a rich intermolecular dynamics in those complexes. The results seem to indicate that complexation with water switches the configuration of formanilide from trans , which is more stable for the bare monomer, to cis , which is more stable for the hydrated complexes. Different interactions of water and formanilide were observed. Water reverts the stability of the cis - trans formanilide conformational equilibrium.