Sorbed Cs on vermiculitized biotite in saturation amount was desorbed by extractions with 1M K+ and Mg2+ at varied temperatures. At room temperature, Cs+ was readily desorbed by leaching with K+ (ca. 100%) but not by Mg2+ (ca. 50%). However, the contrary was true for desorption at high temperatures, in which Mg2+ desorbed far more Cs+ (ca. 100%) than K+ (ca. 25%). XRD analysis of Cs-depleted samples suggested that the contrary abilities of K+ and Mg2+ for Cs desorption at low and high temperatures were attributed to their induced interlayer collapse or expansion during extraction.