The iodine isotopes I-127 and I-129 were isolated from marine algal samples to obtain I-129/I-127 ratios by accelerator mass spectrometry (AMS). The collection efficiency of iodine was 16%-99%, and the treatment temperature in the present study was lower than that used in conventional methods for I-129 analysis. Using the separation method reported here, we can easily measure ultra-low I-129/I-127 ratios in the orders of magnitude of 10(-13)-10(-12), which were observed in algal samples from the pre-nuclear era (before 1945). The I-129/I-127 ratios observed in algae collected from 1929 to 1987 ranged from 10(-13) to 10(-10). The I-129/I-127 ratio in algae collected in 1987 was 100-1000 times as high as the ratio measured in algae collected before 1945. This dramatic increase in I-129/I-127 ratio after 1945 is attributed to human activity. (C) 2012 Elsevier B.V. All rights reserved.