Abstract The microstructure of the normal micelles formed by dimeric surfactants with long spacers, [Br(CH3)2N+(CmH2m+1)-(CH2)S-(CmH2m+1)N+(CH3)2Br, m=10 and s=8, 10 and 12], has been investigated by small-angle neutron scattering and compared with previously reported results for micelles of the same dimeric surfactants with shorter spacers (m=10 and s=2, 3, 4 and 6). It was found that for dimeric surfactants with long spacers (s=8 and 10), both micellar growth and variation in shape occur to only a small extent, if at all, compared with dimeric surfactants with short spacers. However, for the dimeric surfactant with the longest spacer, s=12, the extent of micellar growth and shape variation is also large. These results are due to the differences in conformation of dimeric surfactants with short spacers (s=26) compared with that of the surfactants with long spacers (s=812).