The hydrophilic-hydrophobic diblock copolymer (2-hydroxyethyl methacrylate, HEMA)(41)-block-(isoprene)(84) was synthesized by anionic living polymerization using a protection technique for the hydroxyl group of HEMA. The surface structures of the block copolymer film under dry and wet conditions were analyzed by transmission electron microscopic observation and contact angle and angle-dependent X-ray photoelectron spectroscopic measurements. The top surfaces of the films cast from DMF/THF and THF/methanol are covered with a polyisoprene microdomain. When the as-cast film is exposed to water, the polyisoprene layer at the top surface disappears and is replaced with a PHEMA microdomain. In particular, the hydrated surface clearly emerges under transmission electron microscopy (TEM) by the double-staining method and reverts again to a hydrophobic polyisoprene layer by annealing the film at 52 degrees C for 1 h. Such surface rearrangement occurs repeatedly by soaking in water and annealing. The short-time-scale reorientation of the surface microdomain of the film under dry to wet conditions is successively observed with TEM. The difference between the surface rearrangements of the PHEMA-block-polyisoprene and PHEMA-block-polystyrene films in response to environmental change is also investigated.