Some crystalline salicylideneanilines (anils) undergo reversible colour change when irradiated with ultraviolet light (photochromy) . This phenomenon is largely topochemically dominated since : (i) no correlation exists between photosensitivity and the chemical nature of the ring-substituents, and (ii) polymorphic forms of a given anil differ markedly in light-sensitivity.Absorption spectra measured in solid films of several (monomorphic and dimorphic) anils in temperature ranges from -180" to near the melting points suggest the following classification : a-type ; photochromic, pale yellow a t all temperatures, not thermochromic : P-type ; not photochromic, colour varies with temperature, thermochromic.Further experimental data include the crystallographic constants of 15 anils in their various polymorphic forms, the fluorescence spectra of thermochromic and photochromic crystals, and the thermal fading rates of several photo-coloured materials, including O-deuterated anils.HAVING discussed in previous communications of this series the geometry of irreversible reactions in the solid state (dimerisation, isomerisation) we proceed to analyse in this and subsequent Papers the mechanism of reversible reactions. As example of the latter type of system we have chosen the photochemistry of anils of o-hydroxybenzaldehydes, partly because of the large amount of information available on the polymorphism and photo-behaviour of these compounds (see early summaries 1*2), and partly because of the ease with which they can be prepared. Preliminary accounts of our work on this system have been pre~ented.~" Photochromy.-Salicylideneanilines (I) (referred to as '' anils " in this Paper; and for convenience of discussion locants in the aniline ring are unprimed, those in the salicylidene ring are primed, when the term " a n i l " is used) are pale yellow to dark orange when freshly prepared. Some of the pale yellow anils are sensitive to ultraviolet light (eg., R' * N a 3' OH3655 A) in which their colour deepens to red or brown; certain anils exist in more than one crystal modification,not all of which are necessarily photochromic. Previous work6" has shown, and we have confirmed, that the colour change in photochromic crystals is not accompanied by any observable changes in the X-ray diffraction pattern and infrared (i.r.) spectrum of the crystal. Many previous attempts have been made to correlate photochemical activity with other properties of the members of this family of compounds; no definite correlation has as yet been established.The photo-colour can be "eradicated" by heat or irradiation with visible light (e.g., 4358 A). Fading rates are characteristic for each compound and, wherever polymorphism occurs, for each (photochromic) crystal modification. Stobbe had found that certain systems (fulgides, hydrazones, and stilbene derivatives) were photochromic 5' 2 R2* Part V, preceding paper.