The weight of PCB represented by each electron capture gas chromatographic (EC-GC) peak in solutions of Aroclors 1221–1260 has been determined. The Aroclor samples from which these solutions were prepared are proposed as quantitative PCB standards. Their compositions were determined by elemental analysis, GC with a Coulson conductivity detector, and combined GC/MS. Retention times relative to p,p′-DDE are recommended to designate individual GC peaks of PCB's. A table is given for each Aroclor showing the weight percent of each EC-GC peak in the mixture. A procedure using Aroclors 1242, 1254, and 1260 is recommended for analyzing environmental samples containing more than one Aroclor mixture. Stock solutions of the Aroclors in isooctane are stable except when directly exposed to sunlight. Ampoules of the Aroclor solutions are offered.