Aminosiloxanes bearing 4-amino-3,3-dialkylbutyl substituents are prepared by a multistep process including addition of 2,2-dialkylacetonitrile to vinylic siloxanes, hydrogenation of the nitrile adducts so formed to yield 4-amino-3,3-dialkylbutyl siloxanes, and optional equilibration of the latter with other siloxane sources, to form aminosiloxanes, which are useful for the treatment of textiles and fabrics and for the preparation of siloxane-containing block copolymers.