High molecular weight samples of random copoly(di-n-alkylsiloxane/di-n-hexylsiloxane)s containing either diethylsiloxane, di-n-butylsiloxane, or di-n-pentylsiloxane comonomeric units have been prepared and investigated with respect to their ability to form a hexagonal columnar mesophase. A random substitution of the polysiloxane main chain has been achieved by cationic ring-opening polymerization of mixed cyclic trimers consisting of di-n-alliylsiloxane and di-n-hexylsiloxane units. Structural irregularities caused by introduction of di-n-alklysiloxane units destabilized the crystalline phase, as demonstrated by a lowering of the transition temperatures and by a decrease of the transition enthalpies. All copoly(di-n-pentylsiloxane/di-n-hexylsiloxane)s retained the ability of the homopolymers to form a columnar mesophase. However, when the difference in alkyl side group length exceeded one carbon atom, the columnar mesophase behavior was not displayed.