The present invention relates to processes and reagents for oligonucleotide synthesis and purification. One aspect of the present invention relates to compounds useful for activating phosphoramidites in oligonucleotide synthesis. Another aspect of the present invention relates to a method of preparing oligonucleotides via the phosphoramidite method using an activator of the invention. Another aspect of the present invention relates to sulfur-transfer agents. In a preferred embodiment, the sulfur-transfer agent is a 3-amino-1,2,4-dithiazolidine-5-one. Another aspect of the present invention relates to a method of preparing a phosphorothioate by treating a phosphite with a sulfur-transfer reagent of the invention. In a preferred embodiment, the sulfur-transfer agent is a 3-amino-1,2,4-dithiazolidine-5-one. Another aspect of the present invention relates to compounds that scavenge acrylonitrile produced during the deprotection of phosphate groups bearing ethylnitrile protecting groups. In a preferred embodiment, the acrylonitrile scavenger is a polymer-bound thiol. Another aspect of the present invention relates to agents used to oxidize a phosphite to a phosphate. In a preferred embodiment, the oxidizing agent is sodium chlorite, chloroamine, or pyridine-N-oxide. Another aspect of the present invention relates to methods of purifying an oligonucleotide by annealing a first single-stranded oligonucleotide and second single-stranded oligonucleotide to form a double-stranded oligonucleotide; and subjecting the double-stranded oligonucleotide to chromatographic purification. In a preferred embodiment, the chromatographic purification is high-performance liquid chromatography.