The electron transfer reactions between pyrimidines, e.g. thymine (Thy), uracil (Ura) and cytosine (Cyt), and tetracyanoethylene (TCNE) in acetonitrile were studied using nanosecond laser flash photolysis techniques. Laser photolysis of a mixed solution of TCNE and pyrimidines led to the formation of the anion radical of TCNE, which has an absorption band at 350-500 nm (λ m a x =430 nm) and decays following second-order kinetics. The electron transfer reaction occurs through the excited triplet states of the pyrimidines, and the bimolecular quenching rate constants are 7.6 10 9 dm 3 mol - 1 s - 1 for thymine and 1.7 10 1 0 dm 3 mol - 1 s - 1 for uracil. However, a ground state complex between cytosine and TCNE was found. This complex has an absorption band with λ m a x at 419, 444 and 472 nm. The photoinduced electron transfer between cytosine and TCNE was not observed on a nanosecond time scale during laser photolysis.