The influence of the chemical structure of siloxane alkyl ammonium bromides on their long-term stability in aqueous solution has been investigated by means of the change of the double layer capacity at the mercury/electrolyte interface and by mass spectroscopic measurements. Generally, the degradation of the surfactants is caused by a hydrolytic cleavage of the siloxane bonds, leading to a loss of surface activity. Such a loss could be detected only in concentrated solutions (10(-1) M), not in those less than or equal to 10(-4) M. In the case of branched trisiloxanes, the successive addition of a few methylene groups to one alkyl chain at the nitrogen atom resulted in a dramatic decrease of the rate of hydrolysis. This effect is consistent with the assumption that polar groups, located at the surface of siloxane micelles, protect the siloxane bonds against hydrolytic attack and that this protection increases significantly with increasing alkyl chain length.