The in vivo application of siRNA depends on its cellular uptake and intracellular release, and this is an unsatisfactorily resolved technical hurdle in medicinal applications. Promising concepts directed towards providing efficient cellular and intracellular delivery include lipophilic chemical modification of siRNA. Here we describe chemistry for the production of modified siRNAs designed to display improved transmembrane transport into human cells while preserving the potency of the RNAi-based inhibitors. We report the synthesis and the biochemical and biophysical characteristics of 2′-O-phenylisobutyryloxymethyl (PiBuOM)-modified siRNAs and their impact on biological activity. In the case of spontaneous cellular uptake of naked PiBuOM-modified siRNA, we observed increased target suppression in human cells relative to unmodified or pivaloyloxymethyl (PivOM)-modified siRNA. We provide evidence of improved spontaneous cellular uptake of naked PiBuOM-modified siRNA and of substantial target suppression in human cells in serum-containing medium.