Alkylammonium ion exchange on mixed-layer minerals gives detailed information about the variation of cation density in succeeding interlayer spaces. Three mixed layer samples from Japan (supplied by Dr. H. Kodama) were in- vestigated. Their nearly 1:1 interstratification is based on regularly alternating high- and low-charged interlayer spaces, which are caused by a regular sequence of polar layers. The cation density in the high-charged interlayer spaces is >0.8 eq/(Si, Al)4Oto. The low-charged interlayer spaces have an average cation density of 0.4 eq/(Si, Al)4Oio and heterogeneous charge distribution. The kind of heterogeneity of the Goto Mine sample differs from that of the Yonago Mine and the Honami Mine samples. The Goto Mine specimen has a rather regular sequence of the low- and high-charged interlayers in proportions close to 0.50:0.50. The other two samples contain interlayer spaces with pronounced unsymmetrical charge distribution. The Yonago Mine sample probably has in random distribution with the polar layers about 10% mica-like layers segregated to packets of three and more layers; the ratio of high-charged interlayers to the low-charged ones is increased to about 0.55:0.45. The Honami Mine sample probably contains isolated mica-like layers or pairs of them. The proportion of the high-charged interlayers is estimated to be about 0.53:0.47 and is lower than determined by Kodama from the glycerolated sample. The samples investigated may be considered as end-members of a series of interstratified specimens which begins with smectites with mixed-layer like charge distribution.