Robust and uniform ultrathin elastomeric films with complete surface coverage were fabricated from poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) functionalized with 2% of maleic anhydride by grafting to a chemically modified silicon surface via epoxy-terminated self-assembling monolayers. We varied the thickness L of the SEES film, from 1.4 to 8.5 nm, to test the limits of the stability of microphase-separated structures under confined conditions. We observed that the in-plane cylindrical/spherical microdomain morphology is similar to the bulk microstructure but it is compressed in vertical direction due to film-air and film-substrate interfacial constrains. Such a microstructure is formed at thicknesses in the range from 2.6 to 9 nm and is perfectly defined at L/L-0 = 0.3 (SEBS interdomain spacing, L-0 = 28 nm). Microphase separation is completely suppressed only for extremely thin films with L/L-0 < 0.08. Unlike physically adsorbed SEES monolayers, which dewet the silicon surface, tethered block copolymer monolayers obtained under identical conditions are very stable even under high shear stresses and at elevated temperatures.