Abstract2‐Methylnaphthoquinone (MQ) has been excited in water with a 20 ns laser flash at 353 nm and the resultant transient species have been observed optically. Triplet‐state MQ (3MQ) decays on a sub‐microsecond time scale. It has been characterized in terms of its absorption spectrum and quantum yield. Rate constants have been measured for the decay of3MQ in infinitely dilute solution, for self‐quenching by ground‐state MQ, and for reactions of3MQ with oxygen, thymine, uracil, 6‐methyluracil, and orotic acid. The interaction of3MQ with pyrimidines involves charge transfer to give the pyrimidine cation radical and the MQ : anion radical. These reactions are discussed in relation to the mechanism of pyrimidine photooxidation sensi