By attaching Eu(III)/beta-diketone complex covalently to oligonucleotide, microenvironments at a specific site in DNA have been probed. The conjugate efficiently emits luminescence, since coordination of water to the Eu(III) is competitively inhibited by the internal phosphodiester groups. In the presence of complementary DNA, however, this inhibition is diminished because of rigidity of double-stranded DNA and the emission becomes much weaker. The emission intensity is also affected by other environmental changes.