Data are presented on PCDD/F persistence in a sludge-amended soil sampled from a long-term field experiment started in 1968. Over 50% of the PCDD/Fs present in the soil in 1972 were still present in 1990. The concentrations of all congeners were observed to decrease gradually and in the same manner over this time, indicating that either physical loss of material from the experimental plot had occurred or all congeners had undergone a uniform reduction in extractability over time. Half-lives for the disappearance of PCDD/Fs from the sludge-amended plot post-1972 were of the order of 20 years; however, the degradation/alteration of PCDD/Fs in soil may take much longer since these half-lives are believed to be principally affected by physical removal.