The structures of tetra-n-butylammonium bromide,N(C4H9)4Br (abbreviated: 4Br) and tetra-n-butylam-monium iodide, N(C4H9)4I (abbreviated:4I) have been determined. For 4Br:Mw= 322.4 Da,c-centered monoclinic space group C2/c,a= 14.409(5) Å,b= 13.901(2)Å,c= 20.345(9)Å, β= 110.18(3)Å,V=3729.4Å3,Z=8,Dc= 1.148 g/cm3,F(000)= 1392,R=0.098 for 1311 reflections. For 41:Mw= 369.4 Da, c-centered monoclinic space group C2, α = 14.482(4)Å,b= 14.318(4)Å,c= 19.975(7)Å, β = 110.86(2)°,V=3870Å3, Z = 8,Dc= 1.230 g/cm3,F(000) = 1536,R= 0.142 for 1899 reflections. The structure analysis shows that the n-butyl chains are distorted to different degrees from their all-transconformations and are connected through the quaternary nitrogen with one or twogauche- bonds. The plastic crystalline mesophase for 41 seems to be possible because of a lower packing fraction that produces larger deviations of the bond-rotation angles from the intramolecular energy-minimum, not found for 4Br. The condis crystal mesophase for4Br, in turn, may be due to a more intermeshing cation structure, possible because of the higher packing fraction with the Br−anion. Higher homologues of the tetra-n-alkylammonium salts form only condis-crystal mesophases, lower homologues, only plastic-crystal mesophases. The low-temperature crystal structures of all salts of this homologous series contain similar cation conformations. Then-butyl and longer alkyl chains form layer-like crystals with methyl surfaces. The somewhat poor fit of computed and experimental structures does not arise from experimental error, but is due to static packing disorder.