Résumé : Aims. We study the close circumstellar environment of the nearby S-type star φ1 Gruis using high spatial-resolution, mid-infrared observations from the ESO/VLTI.Methods. Spectra and visibilities were obtained with the MIDI interferometer on the VLT Auxiliary Telescopes. The cool M5III giant Gruis was used as bright primary calibrator, and a dedicated spectro-interferometric study was undertaken to determine its angular diameter accurately. The MIDI measurements were fitted with the 1D numerical radiative transfer code DUSTY to determine the dust shell parameters of Gruis. Taking into account the low spatial extension of the model in the 8-9 m spectral band for the smallest projected baselines, we consider the possibility of a supplementary molecular shell.Results. The MIDI visibility and phase data are mostly dominated by the spherical 21mas (694 ) central star, while the extended dusty environment is over-resolved even with the shortest baselines. No obvious departure from spherical symmetry is found on the milliarcsecond scale. The spectro-interferometric observations are well-fitted by an optically thin ( in the band) dust shell that is located at about 14 stellar radii with a typical temperature of 700 K and composed of 70% silicate and 30% of amorphous alumina grains. An optically thin ( in the band) HO+SiO molecular shell extending from the photosphere of the star up to 4.4 stellar radii with a typical temperature of 1000K is added to the model to improve the fit in the 8-9 m spectral band. We discuss the probable binary origin of asymmetries as revealed by millimetric observations.. © 2008 ESO.