Disclosed are compositions and methods for the preparation of RNA libraries for sequencing, gene expression profiling, microarray and other uses and for simplification of the library preparation process. The disclosure provides blocking oligonucleotides which bind to byproduct nucleic acid molecules formed during the ligation of adapters to nucleic acid segments prior to sequencing and inhibit or block amplification of the byproduct nucleic acid molecules in subsequent amplification reactions. Methods for library preparation using blocking oligonucleotides are also provided.