1. Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK
   
2. Department of Chemistry, University of Oxford, Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK
   
3. Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, USA
   
4. These authors contributed equally to this work.

Correspondence to: Christiane R. Timmel¹P. J. Hore² Correspondence and requests for materials should be addressed to P.J.H. (Email: peter.hore@chem.ox.ac.uk) or C.R.T. (Email: christiane.timmel@chem.ox.ac.uk).

Approximately 50 species, including birds, mammals, reptiles, amphibians, fish, crustaceans and insects, are known to use the Earth's magnetic field for orientation and navigation¹. Birds in particular have been intensively studied, but the biophysical mechanisms that underlie the avian magnetic compass are still poorly understood. One proposal, based on magnetically sensitive free radical reactions^{2, 3}, is gaining support^{4, 5, 6, 7, 8, 9, 10, 11} despite the fact that no chemical reaction in vitro has been shown to respond to magnetic fields as weak as the Earth's (50 T) or to be sensitive to the direction of such a field. Here we use spectroscopic observation of a carotenoid–porphyrin–fullerene model system to demonstrate that the lifetime of a photochemically formed radical pair is changed by application of 50 T magnetic fields, and to measure the anisotropic chemical response that is essential for its operation as a chemical compass sensor. These experiments establish the feasibility of chemical magnetoreception and give insight into the structural and dynamic design features required for optimal detection of the direction of the Earth's magnetic field.