The addition of small amounts (less than 10%) of water to liquid oligomeric triblock copolymers (PEP) of ethylene oxide (E) and propylene oxide (P) increases the viscosity, eta, above that of pure liquid oligomers. Comparison with the concentration dependences of viscosity, eta(c), for oligomeric homopolymers leads to the conclusion that the unusual shape of the dependences for PEP copolymers is due to the interaction of water molecules with hydrophobic propylene oxide units.