There has been a great deal of interest in the research of cellular oxidative stress and particularly the molecules which could alleviate the damage it can cause. Molecular species causing oxidative stress are the reactive oxygen (ROS) and nitrogen species (RNS), such as superoxide anion (O2 •-), peroxynitrite (ONOO-) or hydroxyl radical (OH•). A class of enzymes called the superoxide dismutases (SOD), catalyzing the reaction 2O2 •- + 2H+ → H2O2 + O2, constitutes an effective cellular protection from oxidative damage. Macrocyclic complexes of transition metals have been developed as potential redox-active therapeutics able to neutralize O2 •-. The compounds from the class of manganese porphyrins (MnPs) have been shown to be particularly effective, due to their unique electronic properties enabling them to construct redox cycles that involve the consumption of superoxide and NO-derived oxidants at the expense of readily available biochemical reductants like glutathione, tetrahydrobiopterine or ascorbic acid. The increased activity of MnPs has been achieved by attenuating the metal-centered redox potential using various substituents to the porphyrin ring. The alkylation of the meso(ortho-tetrapyridyl) MnP yielded the redox potential close to optimal and one of the most active compounds was found to be the meso(ortho-tetraethylpyridyl) derivative, MnTE-2-PyP. Besides the appropriate redox potential, the efficiency of MnPs as scavengers of reactive species is influenced by their lipophylicity. Changing the position of alkyl substituents from ortho- to meta- positions on the porphyrin ring increases lipophilicity and new meso(meta-tetraalkylpyridyl) derivatives have been synthesized recently, including meso(meta-tetraethylpyridyl) derivative, MnTE-3-PyP. In the first part of this work, acid-base properties of meso(ortho- i meta-tetraethylpyridyl) MnPs have been investigated. In aqueous solutions MnP complexes can coordinate two axial water molecules which can deprotonate and have a significant influence on the redox potential of MnPs and therein their citoprotective activity. The following species have been confirmed to exist in aqueous solutions in the pH range 2-13: (H2O) Mn(II)TE-m-PyP4+, (HO)Mn(II)TE-m-PyP3+, (H2O)2Mn(III)TE-m-PyP5+, (H2O)(HO)Mn(III)TE-m-PyP4+, (O)(H2O)Mn(III)TE-m-PyP3+, (O)(H2O)Mn(IV)TE-m-PyP4+ and (O)(HO)Mn(IV)TE-m-PyP3+ (m =2, 3).. Using the combined potentiometric and spectrophotometric measurements all dissociation constants connecting these species have been determined, as well as the thermodynamic parameters of dissociation reactions. Considering that the redox potential of MnPs is crucial for their biological activity, in the second part of this work the electrochemical properties of meso(ortho- and meta-tetraethylpyridyl) MnPs have been investigated. The absolute formal potentials and the formal potentials vs. standard hydrogen electrode have been determined by cyclic voltammetry in the pH range 2-13 and the thermodynamic parameters of these formal potentials have been determined as well. Additionally, the electrochemical parameters of these electron transitions have been determined and confirmed by computer simulations. Chronocoulometric measurements enabled the determination of the diffusion coefficients of the MnP species and the sizes of their water cavities have been estimated. Finally, a complete scheme describing the behaviour of MnP complexes in aqueous solutions in the pH range 2-13 has been constructed, taking into account the dissociation and redox equilibria.