A low-spin, macrocyclic iron(II) complex in an aqueous solution responds to the addition of a chemical reactant (dithionite) by transformation into a high-spin complex, detectable by measurement of the longitudinal relaxation time (T(1)) of surrounding water hydrogen nuclear spins. The initial compound does not modify T(1) of pure water at concentrations as high as 4 mM. The response is pH-dependent, and the complex is robust at a variety of conditions.