4-Chloro- and 4-bromo-N-salicylideneaniline form two series of solid solutions, each of which spans the whole range of composition. One series is based on the structure of the stable crystalline modification of the bromocompound and these solutions are photochromic, non-thermochromic, and non-luminescent (type α). The second series is based on the structure of the stable crystalline modification of the chloro-compound and these solutions are thermochromic, luminescent, and non-photochromic (type β). The factors which determine the relative stabilities of the two series are discussed.