The process of co-condensation of 3-aminopropylmethyldimethoxysilane with α,ω-dihydroxyoligo-dimethylsiloxanes is studied. It is shown that the method proposed allows obtaining aminosiloxanes with a definite structure in quantitative yield. The methoxy group in aminopropylmethoxysiloxane oligomers can be quantitatively replaced by trimethylsiloxy group by transesterification with trimethylsilanol, leading to aminosiloxanes with the same chain length. The hydrolysis of the methoxy groups in aminopropylmethoxysiloxane oligomers causes the increase of the polymer chain length due to partial co-condensation of silanol groups leading to aminosiloxanes with a strictly alternating dimethylsiloxane block of the fixed length and a pair of aminopropylmethylsiloxane units. The reaction of N,N’-bis(trimethylsilyl)carbodiimide or hexamethyldisilazane with residual silanol groups leads to trimethylsiloxy-substituted polyaminopolysiloxanes. The trimethylsililation of the residual silanol groups with hexamethyldisilazane is accompanied by the trimethylsililation of the amino groups.