Many spin-crossover complexes exhibit high sensitivity to small environmental changes. Here, we demonstrate proof-of-concept exploitation of this property to report anion binding events in solution. Dichloromethane solutions of the cationic receptor complex [Fe(H(2)bip)(3)](BPh4)(2) (bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine) (1) undergo subtle color changes at room temperature when interrogated by anions capable of hydrogen bonding interactions. Significantly, at -40 degrees C, these secondary binding interactions cause a change from a high- to low-spin state that is qualitatively linked to the strength of the host-guest interaction. The anion recognition property is combined with spin-state switching in a synergistic fashion and offers the possibility of using magnetometry to report host-guest interactions.