The unimolecular dissociation reactions of energy selected molecular ions have been investigated by means of various versions of the photoelectron photoion coincidence technique (TPEPICO, ZEKE-PEPICO). Particular emphasis of this work lies on the investigation of the influence of the external rotation on the dissociation energy of molecular ions. Simple bond fission reactions, e.g. H loss reaction from CH⁴⁺ ions and H loss reaction from C₂H₂⁺ ions, exhibit a strong variation of the dissociation energy with the rotational temperature, i.e. the average rotational angular momentum J of the molecules. Concerted elimination reactions, e.g. the H₂ loss reaction from C₂H₆⁺ ions and H₂ loss reaction from C₂H₄⁺ ions do not show a significant J dependence. This can be rationalized by a simple model according to which the J dependence of the dissociation energy reflects the properties of the rate determining transition state in the reaction.