A technique using CaCl2 solutions for separating seed of approximately equal size and/or shape by relative density was developed using nine hard-seeded weed species and four soybean [Glycine max (L.) Merr.] cultivars having an average density of 1 g/ml or above. Relative seed densities were determined by the water displacement method. From a seed mixture of nine weed species and soybeans, three groups were obtained: Group I, sicklepod (Cassia obtusifolia L.) and smallflower morningglory [Jacquemontia tamnifolia (L.) Griseb]; Group II, hemp sesbania [Sesbania exaltata (Raf.) Cory], tall morningglory [Ipomoea purpurea (L.) Roth], cypressvine morningglory (Ipomoea quamoclit L.), and ivyleaf morningglory [Ipomoea hederacea (L.) Jacq.]; and Group III, soybeans, balloonvine (Cardiospermum halicacabum L.), and velvetleaf (Abutilon theophrasti Medic). Ipomoea lacunosa L. was divided between Group II and Group III due to the range of seed density within the seedlot. The minimum relative CaCl2 solution densities required to separate these seed were 1.38, 1.30, and 1.25 g/ml for Group I, Group II, and Group III, respectively. Further separation into individual species within a group was accomplished by screening in some cases. Sicklepod was separated effectively with this technique from seed screenings obtained from a commercial seed processing plant. There was no effect on germination from soaking weed seed or soybean for 40 min in the CaCl2 solution.