Abstract: Artificial alteration of thirty-five trioctahedral and one dioctahedral micas by solutions varying in strength from 0·001 to 1·00 molal magnesium sulfate was found to approximate a normal exchange reaction after surface effects are eliminated. The equilibrium constants for the reaction: 1/2 Mg2++K-mica=Mg1/2-mica+K+(vermiculite) range from 0·0001 to 0·0028 and average 0·0010 in value. X-ray diffraction study reveals that iron-rich micas tend to develop a 1:1 mixed-layer biotite-vermiculite structure in weak magnesium sulfate solutions whereas magnesium-rich biotite and phlogopite alter to vermiculite. Mica composition also influences the degree of alteration of mica to vermiculite. High fluorine and octahedral multivalent cation contents tend to retard the reaction whereas high magnesium content and perhaps high calcium contents tend to favor the alteration. The equilibrium constant data indicate that vermiculite and hydrobiotite are more stable than trioctahedral micas in most weathering environments.