The photolytic decomposition of tetraethyl lead (TEL) into the derivatives triethyl lead (TriEL), diethyl lead (DiEL) and into inorganic lead (Pb) in aqueous environment has been examined by using a single‐step dithizone extraction method. In cell‐free medium, TEL decomposes rapidly upon illumination to form TriEL, which accumulates in the medium. DiEL is formed in very small quantities only and without any significant accumulation, whereas the concentration of Pb increases slightly but continuously. In darkness the formation of TriEL amounts, as compared to light, to a maximum of 10 %, and DiEL as well as Pb are below the analytical threshold. There is a considerable loss of the initially added TEL: after 98 hr of incubation 96 % of the total lead are eliminated from the illuminated medium and 99.5 % from the non‐illuminated one. In illuminated tap water, where the creation of TriEL takes place much faster than in cultivation medium, the loss of total lead amounts to only 84 %. The formation kinetics for TriEL during the first 72 hr of incubation were determined as follows: tap water/ light: y = 0.18 x; cultivation medium/light: y = ln(l + 0.924 x); cultivation medium/darkness: y = ln(l + 0.006 x), with y being TriEL as ppm Pb and x the hours of incubation.