Adsorption of linear pentenes in H-ZSM-5 at 323 K is investigated using contemporary static and molecular dynamics methods. A physisorbed complex corresponding to free pentene, a π-complex and a chemisorbed species may occur. The chemisorbed species can be either a covalently bonded alkoxide or an ion pair, the so-called carbenium ion. Without finite temperature effects, the π-complex is systematically slightly more bound than the chemisorbed alkoxide complex, whereas molecular dynamics calculations at 323 K yield an almost equal stability of both species. The carbenium ion was not observed during simulations at 323 K. The transformation from the π-complex to the chemisorbed complex is activated by a free energy in the range of 33–42 kJ/mol. Our observations yield unprecedented insights into the stability of elusive intermediates in zeolite catalysis, for which experimental data are very hard to measure.