We describe an apparatus for dielectric spectroscopy (DS) experiments on ultrathin (<100 nm) molecular films in an ultrahigh vacuum (UHV) environment. The apparatus is capable of measurements of ex situ prepared films, as well as in situ physisorbed or chemisorbed layers. Pressures of 5x10(-10) Torr are typical, and the audiofrequency DS measurements can be performed over a range of temperatures 7-500 K, using a continuous-flow cold finger. The combination of turbomolecular pump, differentially pumped quick-access door, and backfilling with dry N(2) affords vent-exchange-pump cycle times under an hour. The system pressure returns to the 10(-9) Torr scale within 48 h, without a high-temperature bake. Since the cold finger is mounted on a long-stroke translation stage, the apparatus can be attached to other analytical UHV equipment, and the sample may be transferred for additional in situ measurements. Film dielectric properties are probed via coplanar interdigital electrode capacitors, which are microfabricated on fused SiO(2) wafers. Besides permitting the usual equilibrium dielectric measurements, the system also provides a novel way to study adsorption/desorption phenomena and film growth.