Formalin fixation is common procedure to prevent tissue autolysis by crosslinking proteins. Not unexpectedly this affects the relaxation properties of the tissue. In addition, quantitative relaxation time constants of formalin fixed brain tissue show a broad variation across different studies. To investigate the contribution of the formulation of formalin to this variability we investigated MR relaxation times of pure formalin solutions from different vendors and the effect of formalin concentration on MR relaxation times of fixed brain tissue. Our results showed a strong variation of relaxation times depending on the concentration used and more importantly on the vendor specific composition of the formalin solutions.