DNA damage induced by noncatalytic aerobic oxidation of pyridinocyanine dyes is described. The dyes are generated in situ during spontaneous oxidations of tetrakis-and bis(N-methylpyridin-4-ium)alkane salts. The mechanism of aerobic oxidation of the latter compound is proposed, and a rare direct catalyst-free transition from saturated alkane to a gemdiol is demonstrated. Thermal DNA oxidation by cyanine dyes has potential in ROS-based cancer treatment and biomedical research.