The cell-impermeant Magnesium Green™, potassium salt exhibits a higher affinity for Mg2+ (Kd ∼1.0 mM) than mag-fura-2 (Kd ∼1.9 mM) or mag-indo-1 (Kd ∼2.7 mM). Upon binding Mg2+, Magnesium Green exhibits and increase in fluorescence emission intensity without a shift in wavelength with absorption/emission maxima ∼506/531 nm.