Abstract: I.R. band shifts in the 830, 750 and 530 cm−1 regions that occur in interstratified layer silicates on heating were used to characterize the nature of the component layers. Two specimens showed shifts characteristic of 1M illite and montmorillonite. Another specimen was similar to 2M illite even though the proportion of expanding layers was 0·4. The shifts for rectorite and allevardite resembled those of paragonite and 2M illite.