A wide range of vacuum-deposited materials (both organic and inorganic) has previously been shown to exhibit sharply optimized properties (including carrier mobility, smoothness, epitaxial order, electrophotographic performance, photovoltaic and photo-e.m.f. effects, and optical properties) when the condensation (substrate) temperatures are held in a very narrow range near 0.33 of the respective normal boiling points. This relationship held for all the really sharp optima which could be found in an extensive search of the literature. A proposed model for this extraordinarily reproducible effect invoked some kind of bulk structural singularity common to the various films; however, few structural data have been available to support or elaborate on this concept. Moreover, the previous work made few correlations between different properties for any one material.We now report a detailed study of this “critical optimization” effect in 70 nm metal-free phthalocyanine films on glass substrates, an organic being chosen as the model material principally because of its lack of stoichiometric complications. For the critical substrate temperature T0 (278–288 K), sharp singularities were observed in optical absorption and reflection, fluorescence, surface smoothness and in reflection high energy electron diffraction (RHEED) data; the structural stability and certain electrical properties also appeared to be optimized. This value of T0 is in excellent agreement with previous work on phthalocyanine films made under experimental conditions which were thoroughly different in almost every way.From a detailed analysis of the data, we conclude that films deposited at T0 possess extraordinarily large crystallites (measured both normal and parallel to the film plane) compared with the other samples. The T0 films, unlike the others, are probably largely monocrystalline normal to the film plane; in plane, the crystallites are large enough to give a spotted RHEED pattern with a 1 mm incident electron beam, which seems remarkable for an unannealed film on an amorphous substrate. Furthermore, the crystallites in films deposited at T0 have substantially different orientations with respect to the substrate plane, compared with the other samples; films formed above and below T0 also possess different orientations from one another. Bulk structural singularities of this kind, if typical of other materials showing critical optimization, constitute a natural common mechanism for the various property singularities and confirm that we are dealing with a specific new ordering phenomenon occurring only at T0.