A detailed kinetic study of the addition of 4-tertiarybutylpyridine to the coordinated dienyl ligand in tricarbonyl (1-5- ɳ-dienyl) iron (II) complexes (1; dienyl = C6H7, 2-MeoC6H6 and C7H9) has been made as a function of temperature and pressure in acetonitrile using stopped-flow techniques. The reactions proceed to completion under pseudo-first- order conditions, i.e. kobs = k1 [4-Me3CPy]. The observed rate sequence C6H7 > 2-Meo C6H6 > C7H9 (e.g. 18:3:1 at 20oC and 0.1 MPa) and the low △ H≠ and large negative △ S≠ and △ V≠ values support direct addition (k1) to the dienyl fragments of complexes (1). The results contrast sharply with the reversible additions of 4-Cyanopyridine, 4- formyldypridine, 2-benzylpyridine and 3-iodopyridine to complexes (1) where kobs was not influenced by pressure changes up to 150 MPa, indicating a zero, volume of activation and the consequent operation of the Odiaka’s “ordered transition state” mechanism.