Quantum chemical calculations of the electronic excited states of a zeaxanthin-chlorophyll dimer show that a zeaxanthin-to-chlorophyll charge-transfer (CT) state lies in the same energy regime as the excited states S-1 of zeaxanthin and Q(y) of chlorophyll, which are relevant for the nonphotochemical quenching (NPQ) process in green plants. Comparison with an analogous violaxanthin-chlorophyll dimer is made, and possible quenching mechanisms via excitation energy transfer or electron transfer are discussed. It is pointed out that the strongly allowed absorption of the zeaxanthin radical cation present in the CT state can be experimentally used as signature of the zea-chl dimer during the NPQ process.