Received December 28, 1981 Accepted for Publication March 23, 1982 The adsorption of gaseous iodine, I2and CH3I, in typical rocks of Japanese ground formation such as granite, tuff and sandstone is described. Adsorption coefficients (Ka) of crushed rock samples were determined by a column technique under dry or wet vapor conditions. The adsorption isotherm was identified as the Langmuir- or Henry-type equation. The Ka value of I2 varied over two orders of magnitude and was 102 to 103 times greater than that of CH3I. The results suggested that the Ka values of I2 and CH3I were proportional to the specific surface areas of crushed rocks and the order of the coefficients was: granite < tuff < sandstone at the same grain size (300-µm diam). The specific surface area of the permeable ground formation was estimated by the Kozeny-Carman equation, consequently, the smallest value of Ka of the rocks was one-tenth to one-thirtieth less than that of crushed rock.