The discovery of a massive planet (51 Peg B) at only 0.05 AU from its star (51 Peg) was a surprising discovery last year. Planets so close to their star had not been predicted by theory and raise new questions on the scenario for the formation of planetary systems. Although simulations have shown that a large gas giant can be stable so close to 51 Peg, we expect a significant evaporation of its atmosphere due to thermal/non-thermal escape and/or gravitational mass loss. The escaping molecules and their dissociation and ionization products can form a large enveloppe around the planet which partially occults the star, since the orbit inclination of 51 Peg B is close to 90 degrees. We propose to search for absorption features of such an enveloppe in the IR spectrum of 51 Peg during the eclipse. We plan to search in the range from 2.38 mu to 3.52 mu and from 4.55 mu to 4.74 mu. These ranges include absorption lines of many potentially present molecules and their daughter products (e.g. H2, He+, CO, CO+, N2+, NH3, CH4,..). The discovery of extra-solar planets is one of the most exciting discoveries in astronomy in this century. At the present stage very little is known about their nature and predictions are rather insecure. We feel that it is strongly required to obtain all information possible about these objects at the available wavelength ranges. ISO can extend these investigations to wavelength ranges not accessible from the ground.