A direct method is described of measuring the mass of substances dissolved from muscovite mica in water. The mica sheets are weighed by the use of a Mettler ultramicrobalance both before and after the immersion in water. The decrease in weight is regarded as identical to the dissolved mass. An analysis by ICP-AES has confirmed that the dissolution is due to an ion exchange of metal ions, mainly potassium ions. As a result of the measurement of the change in weight of mica sheets with immersion time, it has been found that it takes about 30 min to finish the dissolution. The total mass dissolved from basal plane has been determined irrespective of the difference in fraction of metal ions shared with each of the two mica surfaces obtained by cleavage along the basal plane. The measured values of the total mass differ from specimen to specimen and fall in the range from I to 10 mg m(-2). These values of the dissolved mass are comparable to the order of 10 monolayers of potassium ions included in basal plane. The present result suggests that internal metal ions as well as external ions contribute to the ion exchange occurring on the surface of muscovite mica.