This dissertation describes the design and synthesis of a number of porphyrin derivatives designed to model the active sites of the haemoproteins, particularly the natural oxygen-carrier , myoglobin . Previous models, which are reviewed in the text, have usually been derivatives of mesosubstituted porphyrins. In contrast, the porphyrins used in this work were of the natural etio-substituted types, for these are potentially closer mimics of the enzymes' prosthetic groups. Methods are given for the synthesis of porphyrins having two, three , or four propionate sidechains to carry the additional superstructure of the model systems. High yielding routes were developed using dipyrromethene precursors which are now available in large quantities. Several classical procedures in pyrrole chemistry have been improved. The carboxyl functions of the propionic acids have been used to attach a variety of "bridges" of atoms across the face of the macrocycle. The iron adducts of some of these bridged porphyrins have been characterised, and the reaction of the ferrous complexes with oxygen followed by visible spectroscopy. Further developments are described utilising porphyrins having propionate substituents in which a differentiation has been achieved; this allows the synthesis of porphyrins having a bridge and also a sidechain containing an imidazole derivative, to function as a model for the proximal base in myoglobin. Finally, the preparation of a symmetrical doubly-bridged porphyrin is detailed: the iron complexes of these new ligand types were examined. The ferrous derivatives were treated with carbon monoxide and with oxygen to investigate their ability to model myoglobin as oxygen-carriers. Extensive structural studies have been undertaken using proton n.m.r •• The use of deuteriopyridine as a solvent for porphyrin n.m.r. experiments has been examined. It is shown that this solvent has several advantages and that the resultant spectra are readily assigned. A study has been made of the effect of the porphyrions' ring current on the chemical shifts of the protons attached to the bridges. In addition, lanthanide shift reagents and relaxation (T1) measurements were used to probe the detailed structural features of the bridged porphyrins. An attempt is made to unify the results of published model studies in the myoglobin field on the basis of the mechanism of oxidation of ferroporphyrins to their ferric derivatives. This seeks to rationalise the differences which exist between those models derived from porphyrins substituted at the mesa positions and those which have no such substituent. Many of the methods developed for the synthesis and structural analysis are of general applicability in porphyrin work aimed at understanding of the haemenzymes, and suggestions are made concerning future research possibilities.