To understand how chromatin structure is organized by different histone variants, we have measured the genome-wide distribution of nucleosome core particles (NCPs) containing the histone variants H3. 3 and H2A. Z in human cells. We find that a special class of NCPs containing both variants is enriched at'nucleosome-free regions' of active promoters, enhancers and insulator regions. We show that preparative methods used previously in studying nucleosome structure result in the loss of these unstable double-variant NCPs. It seems likely that this instability facilitates the access of transcription factors to promoters and other regulatory sites in vivo. Other combinations of variants have different distributions, consistent with distinct roles for histone variants in the modulation of gene expression.