Single-stranded RNA molecules comprise one or more internal, non-nucleotide spacers, covalently linked with nucleotide portions of the molecule are provided. The single-stranded RNA molecules function as guide or antisense strands that are capable of inhibiting gene expression via an RNA interference mechanism, and thus represent single-stranded RNAi agents. The single-stranded RNAi molecules can be used in methods for a variety of therapeutic, diagnostic, target validation, genomic discovery, genetic engineering, and pharmacogenomic applications.