Ultrathin random copolymer mats with cross-linkable glycidyl methacrylate units, consisting of styrene (S) and 2-vinylpyridine (2VP), are prepared on bare substrates to tune substrate interactions. Lamella-forming polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) films supported by a balanced (or neutral) interfacial interaction, opposite to those supported on a preferentially attracting substrate, exhibit the sensitivity of the order-to-disorder transition (ODT) to slight variations in thin thickness. A neutral composition of the random copolymers is determined to be S = 48.9 mol % for 81 kg/mol PS-b-P2VP films, favoring lamellae oriented perpendicular to the substrate. On the contrary, the lamellae of 15 kg/mol PS-b-P2VP films supported on a neutral mat prefer a parallel orientation to the surface, presumably because such low-molecular-weight PS-b-P2VP films (close to the weak segregation regime) are under the influence of the surface energy difference between the two blocks and a possible chain-end effect. Intriguingly, this effect of neutral mat interaction leads to thickness-dependent ODTs of the 15 kg/mol PS-b-P2VP films. The ODT temperature (Tour) decreases with decreasing film thickness when the film thickness is less than 13L(0) (where L-0 is the lamellar spacing) owing to the substrate compatibility effect of a neutral bottom mat toward the PS-b-P2VP films. Especially for the PS-b-P2VP films confined in the one-neutral and dual-neutral conditions, theoretical calculation based on the discrete chain self-consistent field theory supports that the T-ODT of the films decreases with decreasing film thickness below a certain thickness.